These controllers are responsible for persisting preference values and
managing the visibility and switch state. TetherEnabler will be
listening to changes of shared prefernce values and turn on/off
corresponding type of tethering when tethering is on at that time.
Bug: 146582865
Test: tests are created for each controller class.
Change-Id: I7e601b2bd693da59269517d2988bade7475e7f12