Clean up footer preference on battery saver page.

We set selectable as false in the footer preference library now,
so developers don't need to customize it.

Test: Same behavior on battery saver page.
Bug: 216516546
Change-Id: I188054ef2b317e6df1f54dc89d5c57c644e20ad7

res/xml/battery_saver_settings.xml

@@ -45,8 +45,6 @@
     <com.android.settingslib.widget.FooterPreference
         android:order="100"
         android:key="battery_saver_footer_preference"
-        android:selectable="true"
         android:title="@*android:string/battery_saver_description"
         settings:searchable="false"/>
-
 </PreferenceScreen>

src/com/android/settings/fuelgauge/batterysaver/BatterySaverSettings.java

@@ -83,7 +83,6 @@ public class BatterySaverSettings extends DashboardFragment {
     void addHelpLink() {
         FooterPreference pref = getPreferenceScreen().findPreference(KEY_FOOTER_PREFERENCE);
         if (pref != null) {
-            pref.setSelectable(false);
             pref.setLearnMoreAction(v -> {
                 mMetricsFeatureProvider.action(getContext(),
                         SettingsEnums.ACTION_APP_BATTERY_LEARN_MORE);