Add missing Nullable annotation

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=178117289

library/core/src/main/java/com/google/android/exoplayer2/SimpleExoPlayer.java

@@ -726,7 +726,7 @@ public class SimpleExoPlayer implements ExoPlayer {
   }
 
   @Override
-  public void setPlaybackParameters(PlaybackParameters playbackParameters) {
+  public void setPlaybackParameters(@Nullable PlaybackParameters playbackParameters) {
     player.setPlaybackParameters(playbackParameters);
   }