Fix leak of global references and AudioFocusDeathHandler objects.

Need to call unlinkToDeath() for the object to be garbage collected.
Save the object in the FocusStackEntry, unlink to death when we remove it from the stack.
See http://b/issue?id=5048400

Change-Id: I84c5ba46017d0a8744b5e7509a7c7a5c8dd918fb
author: olivier@google.com