- Use ViewModel to handle the TetheringManager's lifecycle.
- Use LiveData to observe TetheredInterfaces's changes instead of callbacks.
Bug: 246531382
Test: manual test
make RunSettingsRoboTests ROBOTEST_FILTER=TetherSettingsTest
atest -c TetheringManagerModelTest
Change-Id: Ib1a710a19a5340f63401a37b5c2b0bc307ac82f0