Flatten out autofill suggestion generation payload.

The CL modifies the autofill suggestion payload from a nested
std::variant into a flat version with all the atomic classes at the
first level.

This is done in an effort to simplify the logic to make it consistent
and more extendable.

Bug: 324557053
Change-Id: I16f9ec19e707d0ce42212d9da50c9d1742140c91
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5932652
Reviewed-by: Florian Leimgruber <fleimgruber@google.com>
Reviewed-by: Maria Kazinova <kazinova@google.com>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Jan Keitel <jkeitel@google.com>
Commit-Queue: Norge Vizcay <vizcay@google.com>
Cr-Commit-Position: refs/heads/main@{#1369291}