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