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/system/SystemDashboardFragment.java

@@ -101,8 +101,6 @@ public class SystemDashboardFragment extends DashboardFragment {
                 @Override
                 public List<String> getNonIndexableKeys(Context context) {
                     List<String> keys = super.getNonIndexableKeys(context);
-                    keys.add((new BackupSettingsActivityPreferenceController(
-                            context).getPreferenceKey()));
                     keys.add(KEY_RESET);
                     return keys;
                 }