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
diff --git a/res/xml/display_settings.xml b/res/xml/display_settings.xml
index 87d3e5e..aa03bc7 100644
--- a/res/xml/display_settings.xml
+++ b/res/xml/display_settings.xml
@@ -17,6 +17,7 @@
 <PreferenceScreen
     xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:settings="http://schemas.android.com/apk/res/com.android.settings"
+    android:key="display_settings_screen"
     android:title="@string/display_settings"
     settings:keywords="@string/keywords_display">