[iOS] Add constraint to prevent KA height from being too small

On tablets, when using multiple windows, keyboard accessory top
constraints were not sufficient to prevent the keyboard accessory's
height from getting smaller in the out of focus window.

This CL adds a constraint to ensure that the top anchor never
allows the keyboard accessory's height from being reduced under
a certain threshold, allowing it to maintain its height in an
out of focus window.

Bug: 405356632
Change-Id: I06f41e2f9e90117e976315788deff9805e162015
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6388696
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1437422}