Improve flicker when open app info page

The Battery item moves down because the summary of Screen time is
loading in the background, we should put a summary placeholder to
avoid item shifting.

Fixes: 191115152
Test: robotests & visual
Change-Id: Iee297a7350726f02c3569231d37112f8911b1b35

res/xml/app_info_settings.xml

@@ -80,6 +80,7 @@
     <Preference
         android:key="time_spent_in_app"
         android:title="@string/time_spent_in_app_pref_title"
+        android:summary="@string/summary_placeholder"
         settings:controller="com.android.settings.applications.appinfo.TimeSpentInAppPreferenceController" />
 
     <Preference