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/robotests/res/xml-mcc999/location_settings.xml

@@ -17,7 +17,8 @@
 <PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android"
                   xmlns:settings="http://schemas.android.com/apk/res-auto"
                   android:key="fake_title_key"
-                  android:title="screen_title">
+                  android:title="screen_title"
+                  settings:controller="com.android.settings.slices.FakePreferenceController">
 
     <Preference
         android:key="key"