CrOS SplitSettings: Move Google Assistant to OS settings

Fork the search_page so it can be customized for OS settings. Remove
manage search engines. When SplitSettings is enabled:
* Show "Assistant" section in OS settings
* Hide "Assistant" section in browser settings
* Use "Search engine" as the section title in browser settings

Later we may change the "Search engine used in the address bar" label
to make it clear that it also affects the app launcher, but that needs
to go through UX review first.

Bug: 960937
Change-Id: I4b1233c6e0cad2b6bea678eed420d9004434335c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1599928
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658697}