Discard buffer in FakeExoPlayer.

This is in line with a recent change in ExoPlayerImplInternal. Not discarding the buffer
causes OOM when running simulated playbacks.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177573930

testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeAdaptiveMediaPeriod.java

@@ -85,6 +85,14 @@ public class FakeAdaptiveMediaPeriod extends FakeMediaPeriod
   }
 
   @Override
+  public void discardBuffer(long positionUs, boolean toKeyframe) {
+    super.discardBuffer(positionUs, toKeyframe);
+    for (ChunkSampleStream<FakeChunkSource> sampleStream : sampleStreams) {
+      sampleStream.discardBuffer(positionUs, toKeyframe);
+    }
+  }
+
+  @Override
   public long getBufferedPositionUs() {
     super.getBufferedPositionUs();
     return sequenceableLoader.getBufferedPositionUs();

testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeSimpleExoPlayer.java

@@ -364,6 +364,7 @@ public class FakeSimpleExoPlayer extends SimpleExoPlayer {
     public void run() {
       try {
         maybeContinueLoading();
+        mediaPeriod.discardBuffer(rendererPositionUs, /* toKeyframe= */ false);
         boolean allRenderersEnded = true;
         boolean allRenderersReadyOrEnded = true;
         if (playbackState == Player.STATE_READY) {