Fix 5607938 AudioFocusDeathHandler leaks GREF

The AudioFocus death handler was correctly updating the audio
 focus stack when an audio focus client dies, but the death handler
 was leaking GREF if unlinkToDeath() is not called.
The fix consists in making sure unlinkToDeath() is always called
 by calling it in its finalizer.

Change-Id: I0c5343b4986ab582cadbf171fc53816952dc16f5