Improves Settings search result of Flash Notifications

1. Shows "Accessibility > Flash notifications" instead of "Notifications
   > Flash notifications".
2. Hides intro preference in search result.
3. Adds search keywords on Flash Notifications page entry.

Bug: 275649264
Bug: 275553435
Test: searches some queries and checks UI manually
Change-Id: Ie6616b779af77153126846748d1052fb29974e3b