Add PrefController in XML support

Add the ability to define a Preference Controller
in xml using the 'controller' tag.

This is useful for two reasons:
- It allows the controllers to be instantiated via
reflection for Slices and Dashboard fragment
- Removes the requirement that controllers be defined manually
in Fragments

In order to be instantiable, they must have a unified construction
following either:

  ClassName(Context)
  ClassName(Context, String)

Also added a robotest that verifies that all controllers defined
in XML follow the constructor schema, and extend
BasePreferenceController.

Test: robotests
Bug: 67996923
Change-Id: I304b35dc666daebecf0c9e286696f3f2a510704a

src/com/android/settings/system/SystemDashboardFragment.java

@@ -67,7 +67,7 @@ public class SystemDashboardFragment extends DashboardFragment {
 
     private static List<AbstractPreferenceController> buildPreferenceControllers(Context context) {
         final List<AbstractPreferenceController> controllers = new ArrayList<>();
-        controllers.add(new SystemUpdatePreferenceController(context, UserManager.get(context)));
+        controllers.add(new SystemUpdatePreferenceController(context));
         controllers.add(new AdditionalSystemUpdatePreferenceController(context));
         controllers.add(new BackupSettingsActivityPreferenceController(context));
         controllers.add(new GesturesSettingPreferenceController(context));
@@ -88,17 +88,18 @@ public class SystemDashboardFragment extends DashboardFragment {
                 }
 
                 @Override
-                public List<AbstractPreferenceController> getPreferenceControllers(Context context) {
+                public List<AbstractPreferenceController> getPreferenceControllers(
+                        Context context) {
                     return buildPreferenceControllers(context);
                 }
 
                 @Override
                 public List<String> getNonIndexableKeys(Context context) {
                     List<String> keys = super.getNonIndexableKeys(context);
-                    keys.add((new BackupSettingsActivityPreferenceController(context)
-                            .getPreferenceKey()));
+                    keys.add((new BackupSettingsActivityPreferenceController(
+                            context).getPreferenceKey()));
                     keys.add(KEY_RESET);
                     return keys;
                 }
             };
-}
+}