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/SettingsApplication.java

@@ -20,6 +20,7 @@ import android.app.Application;
 
 import com.android.settings.activityembedding.ActivityEmbeddingRulesController;
 import com.android.settings.homepage.SettingsHomepageActivity;
+import com.android.settingslib.applications.AppIconCacheManager;
 
 import java.lang.ref.WeakReference;
 
@@ -44,4 +45,10 @@ public class SettingsApplication extends Application {
     public SettingsHomepageActivity getHomeActivity() {
         return mHomeActivity.get();
     }
+
+    @Override
+    public void onLowMemory() {
+        super.onLowMemory();
+        AppIconCacheManager.getInstance().release();
+    }
 }