Create preference controller for each tethering option

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