Only the first item in Quick Insert grid should be focusable.

As per feedback from accessibility, due to the irregular sizes
of the images in the Quick Insert image grid, tab should just
focus on the first item to avoid confusion.

Although this guidance currently applies only to GIFs, we do this
also for the file image grid because:
- Images may have irregular sizes in the future. This was always
  the plan, but never got to it due to technical limitations.
- There is a bug in the current grid Tab traversal code where the
  user cannot Shift-Tab out of the grid.
- Code is much simpler this way.

Bug: b:388103005
Change-Id: I424a86a6d87044155186b41bb38a83f3a0089729
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6160004
Commit-Queue: Darren Shen <shend@chromium.org>
Auto-Submit: Darren Shen <shend@chromium.org>
Reviewed-by: Michelle Chen <michellegc@google.com>
Cr-Commit-Position: refs/heads/main@{#1403971}