libmediaplayerservice: Explicitly force callbacks to stop running

This is needed to prevent a race condition with
the callbacks running after a close. MediaPlayerService instance
can go out of scope and callbacks from AudioTrack can lead to crash
in MediaPlayerService or NuPlayer.

Flag: EXEMPT bugfix
Bug: 183765091
Bug: 351695164
Test: see bug for validation
Change-Id: I48ebb5643b65eec8751c7bd345bc1266c4a98659