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
382f4e6ea4