[Site search policy] Create featured entries for @ state

This CL creates the TemplateURL entries that will be shown as featured
on the Omnibox when the user types "@".

If an entry is featured by policy, the policy handler creates an extra
TemplateURL with `featured_by_policy` set as true and `@` prepended to
the keyword. Entries with keywords not starting with `@` will always
have `featured_by_policy` set as false. The proposed approach will make
it much easier downstream to handle @ state.

Fixed: 309458462
Bug: 309458462
Change-Id: I651c74a4705cb1229bbab80278e098e031f5ec6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5111621
Reviewed-by: Igor Ruvinov <igorruvinov@chromium.org>
Reviewed-by: manuk hovanesian <manukh@chromium.org>
Commit-Queue: Fabio Tirelo <ftirelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1237821}

• components/search_engines/site_search_policy_handler.cc[diff]
• components/search_engines/site_search_policy_handler_unittest.cc[diff]