BT: Fix mBluetoothLock lock issue

The change fix regression issue of CR-2950986.
1. Change the scope of mBluetoothLock
2. Call unbindService only if mBluetooth is not null.

Change-Id: Ie275e79543cb62e717c8ac4204c6ff37a3552263
CRs-Fixed: 2983815
1 file changed