Fail request for audio focus when linkToDeath fails

When requesting audio focus and linkToDeath() threw a
 RemoteException, AudioService was only logging a warning, and
 was still changing the focus, with a non-null death handler.
The change first check that linkToDeath() didn't fail, and only
 then proceeds to change the focus (dispatch focus change
 notification, updates the focus stack, and updates the remote
 controls).

Change-Id: I0b96dd3c6e924d93f419880349d49f8b360b110b