Bluetooth: Fix to avoid object leaks during BT OFF

A case where script is running to do BT ON/OFF 2000 times,
Each time while turning off BT FULL_CONNECTED_ASYNC_CHANNEL
is disconnected by Pan Service. HALF_CONNECTED_ASYNC_CHANNEL
is not disconnected, which leads memory leak in Pan Service
object as messenger passed by PanService & ASYNC Channel object.
which are still referred from BT tethering framework code. So GC
does not claim for these objects. HALF_CONNECTED_ASYNC_CHANNEL
should be disconnected, once FULL_CONNECTED_ASYNC_CHANNEL
is disconnected to solve this issue.

Change-Id: I5a7a2beb0169a755cf4f3f37fd0a0ab7d08fd9e5
CRs-Fixed: 591180