[iOS] Prevent multiple Password Bottom Sheets from opening simultaneously

On certain websites, multiple focus events on a username field were causing multiple bottom sheets from opening simultaneously. This in turn caused an issue where a released coordinator would cause a view controller's handler to be nil, making any interactions with buttons noops.

The CL fixes this issue by preventing multiple Password Bottom Sheets from opening simultaneously. Note that this measure already exists for Payments Bottom Sheet and had been introduced here:
https://chromium-review.googlesource.com/c/chromium/src/+/4879218

Later on, the following CL refactored the Password Bottom Sheet's dismiss logic, but without adding the check to prevent having multiple simultaneous password bottom sheets:
https://chromium-review.googlesource.com/c/chromium/src/+/5402055

Bug: 351017396
Change-Id: I9421ebd52ece85f0fa08f209aa91114c97a92684
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5677042
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Reviewed-by: Tommy Martino <tmartino@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Federica Germinario <fedegermi@google.com>
Cr-Commit-Position: refs/heads/main@{#1323649}