Check MediaPlayer state, do not teardown() from UI thread.

The teardown() function should only be called from the native
thread when WebCore is happy for us to release the media player.

When we lose audio focus, simply stop playback. Also be careful
not to check the playing state of the MediaPlayer if we are in
the error state.

Bug: 5461143

Change-Id: Iba03bdad5c39b104b3071129430d7ef2177e9358