Apply a new icon cache mechanism for memory improvement

- Avoid loading all app icons at once to decrease memory usage.
- Only load visible icons when entering the apps page.
- Reserve icon placeholder to alleviate icon loading flicker.
- Release icon cache when low memory or leaving apps page.

Bug: 187118427
Bug: 209898662
Test: manual check the smoothness and memory usage of apps pages.
Change-Id: Ifc3c2a73cc88d6e42739df4e8208445afa12e0ea

src/com/android/settings/datausage/UnrestrictedDataAccessPreferenceController.java

@@ -29,6 +29,7 @@ import com.android.settings.applications.AppStateBaseBridge;
 import com.android.settings.core.BasePreferenceController;
 import com.android.settings.dashboard.DashboardFragment;
 import com.android.settings.overlay.FeatureFactory;
+import com.android.settingslib.applications.AppUtils;
 import com.android.settingslib.applications.ApplicationsState;
 import com.android.settingslib.applications.ApplicationsState.AppEntry;
 import com.android.settingslib.applications.ApplicationsState.AppFilter;
@@ -125,6 +126,10 @@ public class UnrestrictedDataAccessPreferenceController extends BasePreferenceCo
             return;
         }
 
+        // Preload top visible icons of app list.
+        AppUtils.preloadTopIcons(mContext, apps,
+                mContext.getResources().getInteger(R.integer.config_num_visible_app_icons));
+
         // Create apps key set for removing useless preferences
         final Set<String> appsKeySet = new TreeSet<>();
         // Add or update preferences