Improve the latency of first entering Apps page

Root cause:
AppsPreferenceController will query the recent app usages twice on
the main thread.

Solution:
Set flag to ensure only triggering refreshUi() once when entering
the Apps page.

Also correct the preference key of apps.xml since we don’t have
duplicated preferences now.

Fixes: 183176038
Test: robotests & visual
Change-Id: Ia41ee1e4a1946b8122deca63e318d3915afcc426

src/com/android/settings/applications/AppDashboardFragment.java

@@ -69,6 +69,7 @@ public class AppDashboardFragment extends DashboardFragment {
         use(SpecialAppAccessPreferenceController.class).setSession(getSettingsLifecycle());
         mAppsPreferenceController = use(AppsPreferenceController.class);
         mAppsPreferenceController.setFragment(this /* fragment */);
+        getSettingsLifecycle().addObserver(mAppsPreferenceController);
 
         final HibernatedAppsPreferenceController hibernatedAppsPreferenceController =
                 use(HibernatedAppsPreferenceController.class);