[omnibox][spark] Add `GetFeaturedKeywordMode()` helper

No behavior change.

Previously, multiple places in the code would compare the user input
with '@' to branch their behavior.

Now, they instead call `AutocompleteInput::GetFeaturedKeywordMode()` to
avoid duplicating that logic.

Bug: 380509146
Change-Id: I505e51bf4a1a5c7acbef66d30fe00aac4ffc3a04
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6101749
Commit-Queue: manuk hovanesian <manukh@chromium.org>
Auto-Submit: manuk hovanesian <manukh@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1401523}