Add some missing Nullable annotations

Also remove NonNull, since we assume NonNull by default. Except
where explicitly overriding a method with NonNull annotated args,
in which case we're still expected to use it.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=209758204

library/core/src/main/java/com/google/android/exoplayer2/mediacodec/MediaCodecRenderer.java

@@ -490,7 +490,7 @@ public abstract class MediaCodecRenderer extends BaseRenderer {
     return codec;
   }
 
-  protected final MediaCodecInfo getCodecInfo() {
+  protected final @Nullable MediaCodecInfo getCodecInfo() {
     return codecInfo;
   }
 

library/core/src/main/java/com/google/android/exoplayer2/trackselection/DefaultTrackSelector.java

@@ -19,6 +19,7 @@ import android.content.Context;
 import android.graphics.Point;
 import android.os.Parcel;
 import android.os.Parcelable;
+import android.support.annotation.NonNull;
 import android.support.annotation.Nullable;
 import android.text.TextUtils;
 import android.util.Pair;
@@ -2032,7 +2033,7 @@ public class DefaultTrackSelector extends MappingTrackSelector {
      *     negative integer if this score is worse than the other.
      */
     @Override
-    public int compareTo(AudioTrackScore other) {
+    public int compareTo(@NonNull AudioTrackScore other) {
       if (this.withinRendererCapabilitiesScore != other.withinRendererCapabilitiesScore) {
         return compareInts(this.withinRendererCapabilitiesScore,
             other.withinRendererCapabilitiesScore);

library/core/src/main/java/com/google/android/exoplayer2/upstream/DefaultHttpDataSource.java

@@ -379,7 +379,7 @@ public class DefaultHttpDataSource extends BaseDataSource implements HttpDataSou
    *
    * @return The current open connection, or null.
    */
-  protected final HttpURLConnection getConnection() {
+  protected final @Nullable HttpURLConnection getConnection() {
     return connection;
   }
 

library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/Cache.java

@@ -15,7 +15,6 @@
  */
 package com.google.android.exoplayer2.upstream.cache;
 
-import android.support.annotation.NonNull;
 import android.support.annotation.Nullable;
 import java.io.File;
 import java.io.IOException;
@@ -96,7 +95,6 @@ public interface Cache {
    * @param listener The listener to add.
    * @return The current spans for the key.
    */
-  @NonNull
   NavigableSet<CacheSpan> addListener(String key, Listener listener);
 
   /**
@@ -113,7 +111,6 @@ public interface Cache {
    * @param key The key for which spans should be returned.
    * @return The spans for the key.
    */
-  @NonNull
   NavigableSet<CacheSpan> getCachedSpans(String key);
 
   /**

library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/SimpleCache.java

@@ -17,6 +17,7 @@ package com.google.android.exoplayer2.upstream.cache;
 
 import android.os.ConditionVariable;
 import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 import android.util.Log;
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.util.Assertions;
@@ -224,7 +225,7 @@ public final class SimpleCache implements Cache {
   }
 
   @Override
-  public synchronized SimpleCacheSpan startReadWriteNonBlocking(String key, long position)
+  public synchronized @Nullable SimpleCacheSpan startReadWriteNonBlocking(String key, long position)
       throws CacheException {
     Assertions.checkState(!released);
     SimpleCacheSpan cacheSpan = getSpan(key, position);

library/hls/src/main/java/com/google/android/exoplayer2/source/hls/playlist/DefaultHlsPlaylistTracker.java

@@ -155,7 +155,7 @@ public final class DefaultHlsPlaylistTracker
   }
 
   @Override
-  public HlsMasterPlaylist getMasterPlaylist() {
+  public @Nullable HlsMasterPlaylist getMasterPlaylist() {
     return masterPlaylist;
   }
 

library/ui/src/main/java/com/google/android/exoplayer2/ui/PlayerView.java

@@ -597,7 +597,7 @@ public class PlayerView extends FrameLayout {
   }
 
   /** Returns the default artwork to display. */
-  public Drawable getDefaultArtwork() {
+  public @Nullable Drawable getDefaultArtwork() {
     return defaultArtwork;
   }