Remove race condition when stopping FakeExoPlayer.

A message to stop the playback and to quit the playback thread was posted in release().
The stop message removed all other already queued messages which might include
the second message to quit the thread. That led to infinite waiting in the
release method because the playback thread never got the quit signal.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=176997104

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

@@ -166,27 +166,13 @@ public PlaybackParameters getPlaybackParameters() {
 
     @Override
     public void stop() {
-      playbackHandler.post(new Runnable() {
-        @Override
-        public void run () {
-          playbackHandler.removeCallbacksAndMessages(null);
-          releaseMedia();
-          changePlaybackState(Player.STATE_IDLE);
-        }
-      });
+      stop(/* quitPlaybackThread= */ false);
     }
 
     @Override
     @SuppressWarnings("ThreadJoinLoop")
     public void release() {
-      stop();
-      playbackHandler.post(new Runnable() {
-        @Override
-        public void run () {
-          playbackHandler.removeCallbacksAndMessages(null);
-          playbackThread.quit();
-        }
-      });
+      stop(/* quitPlaybackThread= */ true);
       while (playbackThread.isAlive()) {
         try {
           playbackThread.join();
@@ -525,6 +511,20 @@ private void releaseMedia() {
       }
     }
 
+    private void stop(boolean quitPlaybackThread) {
+      playbackHandler.post(new Runnable() {
+        @Override
+        public void run () {
+          playbackHandler.removeCallbacksAndMessages(null);
+          releaseMedia();
+          changePlaybackState(Player.STATE_IDLE);
+          if (quitPlaybackThread) {
+            playbackThread.quit();
+          }
+        }
+      });
+    }
+
   }
 
 }