Misc clean up in search

- Remove updateFromClassNameResource() because it's just a no-op method.
- Simplify SearchIndexResources
- Update some pref pages to have unique keys.

Change-Id: I51e542d9947aafd046861ce7be0b4c7213fcd01d
Fixes: 64951285
Test: UniquePreferenceKeyTest
Test: robotest
Bug: 67852637

res/xml/system_dashboard_fragment.xml

@@ -16,6 +16,7 @@
 
 <PreferenceScreen
     xmlns:android="http://schemas.android.com/apk/res/android"
+    android:key="system_dashboard_screen"
     android:title="@string/header_category_system">
 
     <Preference