Instantiate pref controllers from xml if it's defined.

- If a <preference> tag also defines a controller, we will try to
  instantiate it before displaying the UI. The same logic is shared by
  BaseSearchIndexProvider so it also drives search suppression.

- If user also defines a list of controllers programatically, the
  programatically created ones takes precedence.

Bug: 73668763
Test: WIP
Change-Id: I7aecec270bcd3af261e012ef1f6995d2a523cfa1

tests/unit/src/com/android/settings/core/PreferenceControllerContractTest.java

@@ -67,7 +67,7 @@ public class PreferenceControllerContractTest {
             }
 
             final List<AbstractPreferenceController> controllers =
-                    provider.getPreferenceControllers(mContext);
+                    provider.getAllPreferenceControllers(mContext);
             if (controllers == null) {
                 continue;
             }