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