Make tether option preferences listen to own SharedPreferences change

The SharedPreferences value could be changed by other classes. This can
help make UI consistent with shared preferences value.

Bug: 148968321
Test: UsbTetherPreferenceControllerTest;
BluetoothTetherPreferenceControllerTest;
WifiTetherDisablePreferenceControllerTest;
CodeInspectionTest.

Change-Id: I11bdad2729e88c07ea1f046d9e95a6a63b398931