[iOS] Fix crash caused by variable going out of scope

The 'params' variable is passed to the PasswordSuggestionBottomSheetMediator as a const reference, so we can't assume that it'll still exist by the time the completion block of the retrieveSuggestionsForForm method is called, so we store the members we need from it in local variables.

Bug: 346453656
Change-Id: I8f469b938769f74bb721830a7110ed77a67c906a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5622850


Reviewed-by: Tommy Martino <tmartino@chromium.org>
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313547}