Refine WifiTetherSoftApManager and DataSaverBackend to avoid activity leaks

- Declare the service callback as static class and use WeakReference to avoid the callback link being occupied

- Use application context instead of fragment context to avoid context being occupied by external modules

Bug: 237273138
Test: manual test
make RunSettingsRoboTests ROBOTEST_FILTER=TetherSettingsTest
make RunSettingsRoboTests ROBOTEST_FILTER=WifiTetherPreferenceControllerTest

Change-Id: Icca145b8ef08b9949feafbbd4e761f8d50c99181