Add missing @Nullable to MediaCodecAudioRenderer.getMediaClock

Without this @Nullable, potential subclasses can't  override the
method to return null if they don't want to use the renderer as a
media clock.

Issue:#6792
PiperOrigin-RevId: 286545736

RELEASENOTES.md

@@ -16,6 +16,9 @@
     `SsaStyle$SsaAlignment`
     ([#6771](https://github.com/google/ExoPlayer/issues/6771)).
 * Fix `CacheDataSource` to correctly propagate `DataSpec.httpRequestHeaders`.
+* Add missing @Nullable to `MediaCodecAudioRenderer.getMediaClock` and
+  `SimpleDecoderAudioRenderer.getMediaClock`
+  ([#6792](https://github.com/google/ExoPlayer/issues/6792)).
 
 ### 2.11.0 (2019-12-11) ###
 

library/core/src/main/java/com/google/android/exoplayer2/audio/MediaCodecAudioRenderer.java

@@ -520,6 +520,7 @@ public class MediaCodecAudioRenderer extends MediaCodecRenderer implements Media
   }
 
   @Override
+  @Nullable
   public MediaClock getMediaClock() {
     return this;
   }

library/core/src/main/java/com/google/android/exoplayer2/audio/SimpleDecoderAudioRenderer.java

@@ -218,6 +218,7 @@ public abstract class SimpleDecoderAudioRenderer extends BaseRenderer implements
   }
 
   @Override
+  @Nullable
   public MediaClock getMediaClock() {
     return this;
   }