[omnibox] On @-prefixed inputs, prevent most match types from defaulting

This CL changes the allowed_to_be_default_match setting procedure
for matches to disallow defaulting when the input starts with '@',
except in the case of search-what-you-typed matches. This is a bug
fix affecting usability of starter pack instant keyword mode. The
change is applied to matches instead of the providers themselves
because many providers work with "fixed up" inputs that treat extra
symbols as noisy typos to be ignored. Changing that could be
disruptive. We generally do want to find what the user meant to type,
and with this fix we still will but the matches will not be
defaulted in a way that distracts users from the special '@' UX.

Bug: 332416052, 332783748
Change-Id: Id35a13e5e98ab61c2f9670b2fe835b92c97014f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5414576


Reviewed-by: manuk hovanesian <manukh@chromium.org>
Reviewed-by: Orin Jaworski <orinj@chromium.org>
Commit-Queue: Orin Jaworski <orinj@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283397}