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

src/com/android/settings/bluetooth/BluetoothDetailsMacAddressController.java

@@ -45,14 +45,18 @@ public class BluetoothDetailsMacAddressController extends BluetoothDetailsContro
     protected void init(PreferenceScreen screen) {
         mFooterPreference = mFooterPreferenceMixin.createFooterPreference();
         mFooterPreference.setTitle(mContext.getString(
-                    R.string.bluetooth_device_mac_address, mCachedDevice.getAddress()));
+                R.string.bluetooth_device_mac_address, mCachedDevice.getAddress()));
     }
 
     @Override
-    protected void refresh() {}
+    protected void refresh() {
+    }
 
     @Override
     public String getPreferenceKey() {
+        if (mFooterPreference == null) {
+            return null;
+        }
         return mFooterPreference.getKey();
     }
 }