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;

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

@@ -26,7 +26,6 @@ import com.android.settings.core.BasePreferenceController;
 import com.android.settings.overlay.FeatureFactory;
 import com.android.settings.utils.AnnotationSpan;
 import com.android.settingslib.HelpUtils;
-import com.android.settingslib.widget.FooterPreference;
 
 /**
  * Footer for face settings showing the help text and help link.
@@ -42,13 +41,9 @@ public class FaceSettingsFooterPreferenceController extends BasePreferenceContro
         mProvider = FeatureFactory.getFactory(context).getFaceFeatureProvider();
     }
 
-    public FaceSettingsFooterPreferenceController(Context context) {
-        this(context, FooterPreference.KEY_FOOTER);
-    }
-
     @Override
     public int getAvailabilityStatus() {
-        return AVAILABLE;
+        return AVAILABLE_UNSEARCHABLE;
     }
 
     @Override