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