[omnibox][ai-mode] Adjust tab order for RHS AIM button and keyword mode

Desired behavior:

1) `query<tab>` should focus the AIM button.
2) `youtube<tab>` should focus the keyword chip.
3) `youtube<tab><tab>` should focus the 2nd suggestion.
4) `youtube<tab>query<tab>` should focus the 2nd suggestion.
5) `@gemini<tab>` should focus the 2nd suggestion.
6) `<shift+tab xN>` should perfectly undo `<tab xN>`

Currently, (2) inserts the AIM button in the tab order. But the case is
handled correctly anyways because the keyword chip comes earlier in the
tab order. This CL takes a stricter approach of not inserting the AIM
button in the tab order; partly because it's more semantically correct
and partly because it's simpler to implement. No behavior change.

Currently, (3) inserts the AIM button in the tab order. But the case is
handled correctly anyways because the AIM button is hidden after user
tabs into a keyword chip. This CL doesn't insert the AIM button in the
tab order to avoid relying on the AIM button visibility logic remaining
constant over time.

Currently, (5) isn't handled correctly, the AIM button is selected,
breaking muscle memory. This CL doesn't insert the AIM button in the tab
order when the 2nd match has an instant keyword.

Currently, (6) isn't handled correctly when e.g.
`youtube<tab><tab><shift+tab>`. The AIM button isn't focused when
tabbing forward, because the AIM button isn't visible when tabbed into a
keyword chip (see (2) above). But the AIM button is focused when tabbing
backwards because the AIM button is visible when the suggestion below
the keyword chip is focused.

This CL's approach simply avoids adding the AIM button in the tab order
when
- the 1st match has a keyword chip; (2) & (3)
- the 1st match is in typed keyword mode; (4)
- the 2nd match has an instant keyword; (5)

And since this logic doesn't rely on AIM button visibility, (6) works
correctly too.

Bug: 432744091
Change-Id: Ie309867a19f22779689816498c942a06147e291c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6926796
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Commit-Queue: manuk hovanesian <manukh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1515151}