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

src/com/android/settings/wifi/tether/WifiTetherPreferenceController.java

@@ -56,8 +56,9 @@ public class WifiTetherPreferenceController extends AbstractPreferenceController
     WifiTetherSoftApManager mWifiTetherSoftApManager;
 
     public WifiTetherPreferenceController(Context context, Lifecycle lifecycle) {
+        // TODO(b/246537032):Use fragment context to WifiManager service will caused memory leak
         this(context, lifecycle,
-                context.getSystemService(WifiManager.class),
+                context.getApplicationContext().getSystemService(WifiManager.class),
                 context.getSystemService(TetheringManager.class),
                 true /* initSoftApManager */,
                 WifiEnterpriseRestrictionUtils.isWifiTetheringAllowed(context));