Apply top intro preference for face settings page

Do a few things in this cl
- Use correct way to work with controller.
- Refactor xml file.
- Separate content of footer to two parts.
  - First paragraph should become top intro.
  - Rest should keep in footer.

Test: Build apk and see the screen
Bug: 173087905
Change-Id: Icb16dedf6b36542b833527471579aaadb5407d87
Screenshot: https://screenshot.googleplex.com/92Jx6zKyTZU8LJa.png

src/com/android/settings/biometrics/face/FaceSettings.java

@@ -312,7 +312,6 @@ public class FaceSettings extends DashboardFragment {
         controllers.add(new FaceSettingsAppPreferenceController(context));
         controllers.add(new FaceSettingsAttentionPreferenceController(context));
         controllers.add(new FaceSettingsRemoveButtonPreferenceController(context));
-        controllers.add(new FaceSettingsFooterPreferenceController(context));
         controllers.add(new FaceSettingsConfirmPreferenceController(context));
         controllers.add(new FaceSettingsEnrollButtonPreferenceController(context));
         return controllers;