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
Merge "Frameworks: bluetoothmanager avoid NPE" into kk4.4