NotificationPlayer: release MediaPlayer on error and exception

Add a lock to synchronize changes to mPlayer in the playback
  thread and callback thread.
Refactor the exception handling CreationAndCompletionThread to
  always release the player that was set up to play the sound.
In the error and completion callbacks release the media player.

Bug: 110021815
Test: play notifications with wrong URI, verify players are released.

Change-Id: Ibbd06a64c8211dff24b4cfc5960d017721eca123