[iOS]Close add-account coordinator if sign-in is disabled

Signin can be disabled any time in another profile. Indeed, the
sign-in process only block the current profile’s scene.  Since
disabling sign-in is application wide, the account currently being
added can’t be used to signed-in. Admittedly, as it’s added in ISL, it
could still be used in other chrome app

Bug: 433463938
Change-Id: Ife80e29e3c4b19fdc93cbfedf3ece78f73d183c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6783446
Auto-Submit: Arthur Milchior <arthurmilchior@chromium.org>
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Arthur Milchior <arthurmilchior@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1494852}