Fix to unbind BluetoothHeadset service

Connections to BluetoothHeadset service isn't unbound when
bluetooth is disabled and this adds to the number of active
connections in system_server and avoids garbage collection
of death monitors. Fix is to unbind when bluetooth is
disabled.

Change-Id: Iaa406146096a4d92b3704c2b53a23d27f4878a03