Add a Preference into "Network & internet" for new tethering UI

Adding a MasterSwitchPreference into "Network & internet" and create
corresponding controller. The preference is hidden by the feature flag.
It does NOT affect the existing UI.

Bug: 146147778
Test: CodeInspectionTest, TetherPreferenceControllerTest,
NetworkDashboardFragmentTest
Change-Id: Ida8e088efbb22d0c983acecbc420672206392dad