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/backup/BackupSettingsActivityPreferenceController.java

@@ -22,31 +22,29 @@ import android.os.UserManager;
 import android.support.v7.preference.Preference;
 
 import com.android.settings.R;
+import com.android.settings.core.BasePreferenceController;
 import com.android.settings.core.PreferenceControllerMixin;
 import com.android.settingslib.core.AbstractPreferenceController;
 
-public class BackupSettingsActivityPreferenceController extends
-        AbstractPreferenceController implements PreferenceControllerMixin {
+public class BackupSettingsActivityPreferenceController extends BasePreferenceController {
+    private static final String TAG = "BackupSettingActivityPC";
+
     private static final String KEY_BACKUP_SETTINGS = "backup_settings";
-    private static final String TAG = "BackupSettingActivityPC" ;
 
     private final UserManager mUm;
     private final BackupManager mBackupManager;
 
     public BackupSettingsActivityPreferenceController(Context context) {
-        super(context);
+        super(context, KEY_BACKUP_SETTINGS);
         mUm = (UserManager) context.getSystemService(Context.USER_SERVICE);
         mBackupManager = new BackupManager(context);
     }
 
     @Override
-    public boolean isAvailable() {
-        return mUm.isAdminUser();
-    }
-
-    @Override
-    public String getPreferenceKey() {
-        return KEY_BACKUP_SETTINGS;
+    public int getAvailabilityStatus() {
+        return mUm.isAdminUser()
+                ? AVAILABLE
+                : DISABLED_UNSUPPORTED;
     }
 
     @Override
@@ -57,4 +55,4 @@ public class BackupSettingsActivityPreferenceController extends
                 ? R.string.accessibility_feature_state_on
                 : R.string.accessibility_feature_state_off);
     }
-}
+}