Distinguish between NAP and PAN role disconnections

Distinguish between NAP and PANU disconnect in BT tethering
and call appropriate functions. We were never disconnecting
NAP role devices.

ToDo: BluetoothService needs to be refactored, its become too big
1) BluetoothAdapter and BluetoothDevice properties need to be moved to separate
classes.
2) BluetoothPanProfile and BluetoothInputDeviceProfile which are handled
by BluetoothService need to be moved to a separate file.
3) Integrate PAN to the profile state machine.

Change-Id: I32a8d274f38c78931434bd9738c8f6570ba89fcf