Demo app repeatedly shows "audio/video not playable" toast

Solved by adding saving reference to previous TrackGroup. Toast only displayed
after TrackGroup changed.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=152242954

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

@@ -113,6 +113,7 @@ public class PlayerActivity extends Activity implements OnClickListener, ExoPlay
   private TrackSelectionHelper trackSelectionHelper;
   private DebugTextViewHelper debugViewHelper;
   private boolean needRetrySource;
+  private TrackGroupArray lastSeenTrackGroupArray;
 
   private boolean shouldAutoPlay;
   private int resumeWindow;
@@ -261,6 +262,7 @@ public class PlayerActivity extends Activity implements OnClickListener, ExoPlay
           new AdaptiveTrackSelection.Factory(BANDWIDTH_METER);
       trackSelector = new DefaultTrackSelector(videoTrackSelectionFactory);
       trackSelectionHelper = new TrackSelectionHelper(trackSelector, videoTrackSelectionFactory);
+      lastSeenTrackGroupArray = null;
       player = ExoPlayerFactory.newSimpleInstance(this, trackSelector, new DefaultLoadControl(),
           drmSessionManager, extensionRendererMode);
       player.addListener(this);
@@ -479,8 +481,10 @@ public class PlayerActivity extends Activity implements OnClickListener, ExoPlay
   }
 
   @Override
+  @SuppressWarnings("ReferenceEquality")
   public void onTracksChanged(TrackGroupArray trackGroups, TrackSelectionArray trackSelections) {
     updateButtonVisibilities();
+    if (trackGroups != lastSeenTrackGroupArray) {
       MappedTrackInfo mappedTrackInfo = trackSelector.getCurrentMappedTrackInfo();
       if (mappedTrackInfo != null) {
         if (mappedTrackInfo.getTrackTypeRendererSupport(C.TRACK_TYPE_VIDEO)
@@ -492,6 +496,8 @@ public class PlayerActivity extends Activity implements OnClickListener, ExoPlay
           showToast(R.string.error_unsupported_audio);
         }
       }
+      lastSeenTrackGroupArray = trackGroups;
+    }
   }
 
   // User controls