Let tether preference controllers extend TogglePreferenceController

Refactored USBTether, BluetoothTether and WifiTetherDisable preference
controllers to extend TogglePreferenceControllers, since they are all
SwitchPreference.

Bug: 150634051
Test: UsbTetherPreferenceControllerTest,
BluetoothTetherPreferenceControllerTest,
WifiTetherDisablePreferenceControllerTest,
CodeInspectionTest

Change-Id: I3b1d1c4e678c65ced8dd506a87c3c43ed300d64f