Bluetooth: only unregisterReceiver if we register

We can get to onDestroy() without registering now.

Test: none
Fixes: 36501241
Change-Id: I90da3b350f9951993238d51f9fde86ccb335cafd
(cherry picked from commit af34ed06f1b4b3734d883c7398b713b20585e1bd)
1 file changed