Frameworks: bluetoothmanager avoid NPE

callback on unregister get sometimes null. Not sure
where it comes from currently due that all instances
calling the unregisterStateChangeCallback are
well. Whatever to prevend the NPE check callback for null
like stated in RemoteCallBackList.java

Change-Id: Ie49c2a507dcecdbdaa155fd1a5d93fc5d6a2baf8