Create a TetherEnabler class to manage tether settings switch

This class is created to manage the switch state of overall tethering
state. It can turn on/off each type of tethering based on stored value
in SharedPreference. Also, it listens to data saver state change.

Bug: 145923107
Test: TetherEnablerTest
Change-Id: I7f360329569f53f34cf13065aa0e00ad9b55f659
2 files changed