Monitor Bluetooth Tethering state and update preference when there is Bluetooth tethering state change. Add test case to verify when recieve BluetoothPan.TETHERING_STATE_CHANGED will update bluetooth state Bug: 138688805 Test: make RunSettingsRoboTests Change-Id: I8f468d1d99ed6d87f6cd8305ef56b0d7a5dec95b