Update state when there is Bluetooth tethering state changed

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
Merged-Id: I8f468d1d99ed6d87f6cd8305ef56b0d7a5dec95b
Change-Id: I8f468d1d99ed6d87f6cd8305ef56b0d7a5dec95b