Make sure SmoothStreaming manifest durations are -1 for Live.

Plus start to properly document the SmoothStreaming package.
Note that where the documentation is a little vague, this is
because the original SmoothStreaming documentation is equally
vague!

library/src/main/java/com/google/android/exoplayer/C.java

@@ -20,6 +20,11 @@ package com.google.android.exoplayer;
  */
 public final class C {
 
+  /**
+   * Represents an unknown microsecond time or duration.
+   */
+  public static final long UNKNOWN_TIME_US = -1;
+
   /**
    * Represents an unbounded length of data.
    */

library/src/main/java/com/google/android/exoplayer/FrameworkSampleSource.java

@@ -71,10 +71,10 @@ public final class FrameworkSampleSource implements SampleSource {
       trackInfos = new TrackInfo[trackStates.length];
       for (int i = 0; i < trackStates.length; i++) {
         android.media.MediaFormat format = extractor.getTrackFormat(i);
-        long duration = format.containsKey(android.media.MediaFormat.KEY_DURATION) ?
-            format.getLong(android.media.MediaFormat.KEY_DURATION) : TrackRenderer.UNKNOWN_TIME_US;
+        long durationUs = format.containsKey(android.media.MediaFormat.KEY_DURATION) ?
+            format.getLong(android.media.MediaFormat.KEY_DURATION) : C.UNKNOWN_TIME_US;
         String mime = format.getString(android.media.MediaFormat.KEY_MIME);
-        trackInfos[i] = new TrackInfo(mime, duration);
+        trackInfos[i] = new TrackInfo(mime, durationUs);
       }
       prepared = true;
     }

library/src/main/java/com/google/android/exoplayer/TrackInfo.java

@@ -20,9 +20,21 @@ package com.google.android.exoplayer;
  */
 public final class TrackInfo {
 
+  /**
+   * The mime type.
+   */
   public final String mimeType;
+
+  /**
+   * The duration in microseconds, or {@link C#UNKNOWN_TIME_US} if the duration is unknown.
+   */
   public final long durationUs;
 
+  /**
+   * @param mimeType The mime type.
+   * @param durationUs The duration in microseconds, or {@link C#UNKNOWN_TIME_US} if the duration
+   *     is unknown.
+   */
   public TrackInfo(String mimeType, long durationUs) {
     this.mimeType = mimeType;
     this.durationUs = durationUs;

library/src/main/java/com/google/android/exoplayer/TrackRenderer.java

@@ -67,9 +67,9 @@ public abstract class TrackRenderer implements ExoPlayerComponent {
   protected static final int STATE_STARTED = 3;
 
   /**
-   * Represents an unknown time or duration.
+   * Represents an unknown time or duration. Equal to {@link C#UNKNOWN_TIME_US}.
    */
-  public static final long UNKNOWN_TIME_US = -1;
+  public static final long UNKNOWN_TIME_US = C.UNKNOWN_TIME_US; // -1
   /**
    * Represents a time or duration that should match the duration of the longest track whose
    * duration is known.

library/src/main/java/com/google/android/exoplayer/chunk/SingleSampleChunkSource.java

@@ -15,6 +15,7 @@
  */
 package com.google.android.exoplayer.chunk;
 
+import com.google.android.exoplayer.C;
 import com.google.android.exoplayer.MediaFormat;
 import com.google.android.exoplayer.TrackInfo;
 import com.google.android.exoplayer.upstream.DataSource;
@@ -42,7 +43,8 @@ public class SingleSampleChunkSource implements ChunkSource {
    * @param dataSource A {@link DataSource} suitable for loading the sample data.
    * @param dataSpec Defines the location of the sample.
    * @param format The format of the sample.
-   * @param durationUs The duration of the sample in microseconds.
+   * @param durationUs The duration of the sample in microseconds, or {@link C#UNKNOWN_TIME_US} if
+   *     the duration is unknown.
    * @param mediaFormat The sample media format. May be null.
    */
   public SingleSampleChunkSource(DataSource dataSource, DataSpec dataSpec, Format format,

library/src/main/java/com/google/android/exoplayer/smoothstreaming/SmoothStreamingManifest.java

@@ -15,6 +15,7 @@
  */
 package com.google.android.exoplayer.smoothstreaming;
 
+import com.google.android.exoplayer.C;
 import com.google.android.exoplayer.util.Assertions;
 import com.google.android.exoplayer.util.Util;
 
@@ -33,28 +34,77 @@ public class SmoothStreamingManifest {
 
   private static final long MICROS_PER_SECOND = 1000000L;
 
+  /**
+   * The client manifest major version.
+   */
   public final int majorVersion;
+
+  /**
+   * The client manifest minor version.
+   */
   public final int minorVersion;
-  public final long timescale;
+
+  /**
+   * The number of fragments in a lookahead, or -1 if the lookahead is unspecified.
+   */
   public final int lookAheadCount;
+
+  /**
+   * True if the manifest describes a live presentation still in progress. False otherwise.
+   */
   public final boolean isLive;
+
+  /**
+   * Content protection information, or null if the content is not protected.
+   */
   public final ProtectionElement protectionElement;
+
+  /**
+   * The contained stream elements.
+   */
   public final StreamElement[] streamElements;
+
+  /**
+   * The overall presentation duration of the media in microseconds, or {@link C#UNKNOWN_TIME_US}
+   * if the duration is unknown.
+   */
   public final long durationUs;
+
+  /**
+   * The length of the trailing window for a live broadcast in microseconds, or
+   * {@link C#UNKNOWN_TIME_US} if the stream is not live or if the window length is unspecified.
+   */
   public final long dvrWindowLengthUs;
 
+  /**
+   * @param majorVersion The client manifest major version.
+   * @param minorVersion The client manifest minor version.
+   * @param timescale The timescale of the media as the number of units that pass in one second.
+   * @param duration The overall presentation duration in units of the timescale attribute, or 0
+   *     if the duration is unknown.
+   * @param dvrWindowLength The length of the trailing window in units of the timescale attribute,
+   *     or 0 if this attribute is unspecified or not applicable.
+   * @param lookAheadCount The number of fragments in a lookahead, or -1 if this attribute is
+   *     unspecified or not applicable.
+   * @param isLive True if the manifest describes a live presentation still in progress. False
+   *     otherwise.
+   * @param protectionElement Content protection information, or null if the content is not
+   *     protected.
+   * @param streamElements The contained stream elements.
+   */
   public SmoothStreamingManifest(int majorVersion, int minorVersion, long timescale, long duration,
       long dvrWindowLength, int lookAheadCount, boolean isLive, ProtectionElement protectionElement,
       StreamElement[] streamElements) {
     this.majorVersion = majorVersion;
     this.minorVersion = minorVersion;
-    this.timescale = timescale;
     this.lookAheadCount = lookAheadCount;
     this.isLive = isLive;
     this.protectionElement = protectionElement;
     this.streamElements = streamElements;
-    dvrWindowLengthUs = Util.scaleLargeTimestamp(dvrWindowLength, MICROS_PER_SECOND, timescale);
-    durationUs = Util.scaleLargeTimestamp(duration, MICROS_PER_SECOND, timescale);
+    dvrWindowLengthUs = dvrWindowLength == 0 ? C.UNKNOWN_TIME_US
+        : Util.scaleLargeTimestamp(dvrWindowLength, MICROS_PER_SECOND, timescale);
+    durationUs = duration == 0 ? C.UNKNOWN_TIME_US
+        : Util.scaleLargeTimestamp(duration, MICROS_PER_SECOND, timescale);
   }
 
   /**