[Wi-Fi] Fix exception when WifiTracker2 callbacks to Wi-Fi Settings

When WifiTracker2 callbacks to Wi-Fi Settings, should check if the
activity is finishing at first.

Bug: 151677440
Test: make RunSettingsRoboTests -j ROBOTEST_FILTER=WifiSettings2Test
Change-Id: I95737eae3f67e6471c75ba845b57af2d721d5d37

src/com/android/settings/SettingsPreferenceFragment.java

@@ -699,4 +699,9 @@ public abstract class SettingsPreferenceFragment extends InstrumentedPreferenceF
         }
         getActivity().setResult(result);
     }
+
+    protected boolean isFinishingOrDestroyed() {
+        final Activity activity = getActivity();
+        return activity == null || activity.isFinishing() || activity.isDestroyed();
+    }
 }