Use FooterPreference in xml explicitly

Removed the FooterPreferenceMixin from the DataSaverSummary page.

Fixes: 139393749
Test: manual test
Test: make RunSettingsRoboTests ROBOTEST_FILTER=com.android.settings.datausage
Change-Id: Ie229acd903eb24e369beef8bbf5ff0bdd9ce50d1

res/xml/data_saver.xml

@@ -27,4 +27,10 @@
         android:fragment="com.android.settings.datausage.UnrestrictedDataAccess"
         settings:controller="com.android.settings.applications.specialaccess.DataSaverController" />
 
+    <com.android.settingslib.widget.FooterPreference
+        android:key="data_saver_footer"
+        android:title="@*android:string/data_saver_description"
+        android:selectable="false"
+        settings:searchable="false"/>
+
 </PreferenceScreen>

src/com/android/settings/datausage/DataSaverSummary.java

@@ -63,8 +63,6 @@ public class DataSaverSummary extends SettingsPreferenceFragment
         super.onCreate(icicle);
 
         addPreferencesFromResource(R.xml.data_saver);
-        mFooterPreferenceMixin.createFooterPreference()
-                .setTitle(com.android.internal.R.string.data_saver_description);
         mUnrestrictedAccess = findPreference(KEY_UNRESTRICTED_ACCESS);
         mApplicationsState = ApplicationsState.getInstance(
                 (Application) getContext().getApplicationContext());