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

tests/robotests/src/com/android/settings/search/XmlParserUtilTest.java

@@ -128,6 +128,16 @@ public class XmlParserUtilTest {
         assertThat(key).isNull();
     }
 
+    @Test
+    @Config(qualifiers = "mcc999")
+    public void testControllerAttribute_returnsValidData() {
+        XmlResourceParser parser = getChildByType(R.xml.about_legal, "Preference");
+        final AttributeSet attrs = Xml.asAttributeSet(parser);
+
+        String controller = XmlParserUtils.getController(mContext, attrs);
+        assertThat(controller).isEqualTo("mind_flayer");
+    }
+
     @Test
     public void testDataSummaryInvalid_ReturnsNull() {
         XmlResourceParser parser = getParentPrimedParser(R.xml.display_settings);