Declare "searchable" attribute for preferences.

Now we can easily mark a preference nonIndexable in xml instead of
adding key into searchIndexProvider.

Bug: 112608186
Test: robotests
Change-Id: I0ff16d44bb7b6ad148d3d35f09ca0da0163f73f4

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

@@ -97,13 +97,5 @@ public class AppAndNotificationDashboardFragment extends DashboardFragment {
                         Context context) {
                     return buildPreferenceControllers(context, null, null /* host */);
                 }
-
-                @Override
-                public List<String> getNonIndexableKeys(Context context) {
-                    List<String> keys = super.getNonIndexableKeys(context);
-                    keys.add((new SpecialAppAccessPreferenceController(context))
-                            .getPreferenceKey());
-                    return keys;
-                }
             };
 }