Bluetooth: Release pan proxy object while coming out from tethering menu

A case where user is going into tethering menu & coming out repeatedly
(2000 iterations) irrespective of Bluetooth state(ON or OFF). Setting
application get crashed after 1386 iterations. Each time TetherSetting
& BluetoothPan object were leaked, because connection of the pan profile
proxy to the Service was not closed by tetherSetting while coming out
from tethering menu. Releasing the pan proxy object while coming out
from tethering menu to avoid memory/objects leak.

Change-Id: I06a5e1ed37a024695fddb1e95f62b72fd56510d3
CRs-Fixed: 622489