Bluetooth:GAP: Handle exception while unregistering the Receiver

Handle the IllegalArgumentException while unregistering the
Receiver.

Usually when user presses both cancel and okay for the disconnect
dialog, OnClickListener will be called twice for both options,
and for the second time it is expected that receiver would have
already unregistered.

Change-Id: I36e30d551df76b7cfd1754305a1087578fd4a275
CRs-Fixed: 595648