[iOS] Dynamic button stack for alert view controller

The media permissions alert controller places buttons horizontally so
that it aligns with the previous UIAlertViewController that asks user's
permission to give Chromium access to microphone and/or camera. However,
the system UIAlertController dynamically places the buttons based on
dynamic text size - if the text size gets large, the buttons will be
placed vertically instead of horizontally to avoid text truncation.

This CL updates AlertViewController so that it does the same.

Video (actually a gif):
https://screenshot.googleplex.com/AzvR358zy7a8WN9

Fixed: 1431854
Change-Id: I841b7e48a4ba619f77def6c75cb6f0e5b753b299
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4416237


Reviewed-by: Scott Yoder <scottyoder@google.com>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Ginny Huang <ginnyhuang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1129261}