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/search/XmlParserUtils.java

@@ -71,6 +71,10 @@ public class XmlParserUtils {
         return getData(context, attrs, R.styleable.Preference, R.styleable.Preference_keywords);
     }
 
+    public static String getController(Context context, AttributeSet attrs) {
+        return getData(context, attrs, R.styleable.Preference, R.styleable.Preference_controller);
+    }
+
     public static int getDataIcon(Context context, AttributeSet attrs) {
         final TypedArray ta = context.obtainStyledAttributes(attrs,
                 com.android.internal.R.styleable.Preference);