Fix crash in battery info settings page.

- when receive package updates, check if the buttons preference is
available before trying to update the UI.

Change-Id: Ie3482b94c84cd55e9a138af4719a031f5167e089
Fixes: 74130214
Test: make RunSettingsRoboTests

src/com/android/settings/fuelgauge/AppButtonsPreferenceController.java

@@ -297,7 +297,9 @@ public class AppButtonsPreferenceController extends AbstractPreferenceController
 
     @Override
     public void onPackageListChanged() {
-        refreshUi();
+        if (isAvailable()) {
+            refreshUi();
+        }
     }
 
     @Override