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/deviceinfo/AdditionalSystemUpdatePreferenceController.java

@@ -17,26 +17,23 @@ package com.android.settings.deviceinfo;
 
 import android.content.Context;
 
+import com.android.settings.core.BasePreferenceController;
 import com.android.settings.core.PreferenceControllerMixin;
 import com.android.settingslib.core.AbstractPreferenceController;
 
-public class AdditionalSystemUpdatePreferenceController extends
-        AbstractPreferenceController implements PreferenceControllerMixin {
+public class AdditionalSystemUpdatePreferenceController extends BasePreferenceController {
 
     private static final String KEY_UPDATE_SETTING = "additional_system_update_settings";
 
     public AdditionalSystemUpdatePreferenceController(Context context) {
-        super(context);
+        super(context, KEY_UPDATE_SETTING);
     }
 
     @Override
-    public boolean isAvailable() {
+    public int getAvailabilityStatus() {
         return mContext.getResources().getBoolean(
-                com.android.settings.R.bool.config_additional_system_update_setting_enable);
+                com.android.settings.R.bool.config_additional_system_update_setting_enable)
+                ? AVAILABLE
+                : DISABLED_UNSUPPORTED;
     }
-
-    @Override
-    public String getPreferenceKey() {
-        return KEY_UPDATE_SETTING;
-    }
-}
+}