Fix 5623252 unlink to death of audio focus client early

When removing an entry in the audio focus stack, don't wait for
 this object's finalizer to unlink to the death of the associated
 client, and unlink right away.
The FocusStackEntry.unlinkToDeath() method is made safe to call
 by catching the exception it can throw inside its body.

Change-Id: Id347a933d72dcb41531adff2854fb00277032cc9