Fix bug 40640864 Use a death handler on audio focus changes from mode

Use a death handler when audio focus changes are caused by audio
 mode changes.

The bug comes from the fact that audio focus clients that use the
 audio mode for automatic focus handling didn't register a death
 handler, which was set to null. When such a client died, this
 handler was compared against the one to remove from the
 audio focus stack, which resulted in an NPE.
The fix consists in registering a valid IBinder object in the
 audio focus stack, even for clients whose focus requests originate
 from a change in audio mode, as implemented in the
 handleFocusForCalls() method.

Change-Id: Id9e1d3d10afcd99969285f6d60fc4d7dde1e4a10