[Settings] Avoid rendering hidden preferences

Under dashboards with expand button, preferences are renderred even
not been expanded.
Avoid from rendering un-expanded preference can improve the performance
of dashboard display.

Bug: 141833767
Test: manual
Change-Id: I00c6f827a0b7b7cec6a6fd8c809b94ca1dce88bb

src/com/android/settings/dashboard/DashboardFragment.java

@@ -319,6 +319,13 @@ public abstract class DashboardFragment extends SettingsPreferenceFragment
         return false;
     }
 
+    /**
+     * Get current PreferenceController(s)
+     */
+    protected Collection<List<AbstractPreferenceController>> getPreferenceControllers() {
+        return mPreferenceControllers.values();
+    }
+
     /**
      * Update state of each preference managed by PreferenceController.
      */