Improve App notification loading

1) Use UiBlocker, for better animations for apps with a lot of channels
2) Only load data in onResume, because the data loading step in some
of the controllers is expensive

Test: make -j64 RunSettingsRoboTests ROBOTEST_FILTER="com.android.settings.notification"
Test: load small channel and large (6000ish) channel app pages
Test: load individual channel pages
Test: load in-app channel shelf UI
Fixes: 215072888
Change-Id: I9a1c96f75b02f94b3ffc529d17d9c0cad7752de1

src/com/android/settings/notification/app/HeaderPreferenceController.java

@@ -45,7 +45,7 @@ public class HeaderPreferenceController extends NotificationPreferenceController
     private boolean mStarted = false;
 
     public HeaderPreferenceController(Context context, DashboardFragment fragment) {
-        super(context, null);
+        super(context, null, PREF_KEY_APP_HEADER);
         mFragment = fragment;
     }
 
@@ -55,8 +55,8 @@ public class HeaderPreferenceController extends NotificationPreferenceController
     }
 
     @Override
-    public boolean isAvailable() {
-        return mAppRow != null;
+    public int getAvailabilityStatus() {
+        return mAppRow != null ? AVAILABLE : CONDITIONALLY_UNAVAILABLE;
     }
 
     @Override