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/language_and_input_new_advance_button.xml

@@ -29,6 +29,7 @@
         android:fragment="com.android.settings.localepicker.LocaleListEditor"/>
 
     <PreferenceCategory
+        android:key="keyboards_category"
         android:title="@string/keyboard_and_input_methods_category">
         <Preference
             android:key="virtual_keyboard_pref"
@@ -42,6 +43,7 @@
     </PreferenceCategory>
 
     <PreferenceCategory
+        android:key="input_assistance_category"
         android:title="@string/input_assistance">
         <!-- Spell checker preference title, summary and fragment will be set programmatically. -->
         <!-- Note: Mark this as persistent="false" to remove unnecessarily saved shared preference.
@@ -64,7 +66,8 @@
             android:title="@string/user_dict_settings_title"/>
     </PreferenceCategory>
 
-    <PreferenceCategory>
+    <PreferenceCategory
+        android:key="pointer_and_tts_category">
 
         <com.android.settings.PointerSpeedPreference
             android:key="pointer_speed"