Add absolute start time to Timeline

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=129869741

demo/src/main/java/com/google/android/exoplayer2/demo/EventLogger.java

@@ -95,9 +95,9 @@ import java.util.Locale;
     boolean isFinal = timeline.isFinal();
     int periodCount = timeline.getPeriodCount();
     int seekWindowCount = timeline.getSeekWindowCount();
-    Log.d(TAG, "sourceInfo[isFinal=" + isFinal + ", periodCount="
-        + (periodCount == Timeline.UNKNOWN_PERIOD_COUNT ? "?" : periodCount) + ", seekWindows: "
-        + seekWindowCount);
+    Log.d(TAG, "sourceInfo[isFinal=" + isFinal + ", startTime=" + timeline.getAbsoluteStartTime()
+        + ", periodCount=" + (periodCount == Timeline.UNKNOWN_PERIOD_COUNT ? "?" : periodCount)
+        + ", seekWindows: " + seekWindowCount);
     for (int seekWindowIndex = 0; seekWindowIndex < seekWindowCount; seekWindowIndex++) {
       Log.d(TAG, "  " + timeline.getSeekWindow(seekWindowIndex));
     }

library/src/main/java/com/google/android/exoplayer2/source/ConcatenatingMediaSource.java

@@ -151,6 +151,11 @@ public final class ConcatenatingMediaSource implements MediaSource {
       return isFinal;
     }
 
+    @Override
+    public long getAbsoluteStartTime() {
+      return timelines[0].getAbsoluteStartTime();
+    }
+
     @Override
     public long getPeriodDuration(int index) {
       int sourceIndex = getSourceIndexForPeriod(index);

library/src/main/java/com/google/android/exoplayer2/source/SinglePeriodTimeline.java

@@ -90,6 +90,11 @@ public final class SinglePeriodTimeline implements Timeline {
     return isFinal;
   }
 
+  @Override
+  public long getAbsoluteStartTime() {
+    return 0;
+  }
+
   @Override
   public long getPeriodDuration(int index) {
     if (index != 0) {

library/src/main/java/com/google/android/exoplayer2/source/Timeline.java

@@ -44,6 +44,11 @@ public interface Timeline {
    */
   boolean isFinal();
 
+  /**
+   * Returns the absolute start time of the timeline in milliseconds.
+   */
+  long getAbsoluteStartTime();
+
   /**
    * Returns the duration of the period at {@code index} in the timeline, in milliseconds, or
    * {@link ExoPlayer#UNKNOWN_TIME} if not known.

library/src/main/java/com/google/android/exoplayer2/source/dash/DashMediaSource.java

@@ -503,6 +503,11 @@ public final class DashMediaSource implements MediaSource {
       return !manifest.dynamic;
     }
 
+    @Override
+    public long getAbsoluteStartTime() {
+      return manifest.availabilityStartTime + manifest.getPeriod(0).startMs;
+    }
+
     @Override
     public long getPeriodDuration(int index) {
       return manifest.getPeriodDurationMs(index);