Update SharedPreference values based on tethering state

TetherEnabler need to update tethering state of each tethering
interfaces to make sure other preferences that depends on these
SharedPreferences be consistent with UI.

Bug: 148968321
Test: TetherEnablerTest; CodeInspectionTest. Manully test, built and
flashed to crosshatch device.

Change-Id: Ie0be7748adf20e6fb0ff5489795b0ca0664b6323