[iOS] Move properties to ivars in AddPasswordViewController

This CL moves properties to ivars in AddPasswordViewController.
Indeed, according to best practices, using ivars is better than private
properties where they simply act as a wrapper around plain ivars.

Fixed: 402199280
Change-Id: Ibdd6b7bb0cf542ae7160edf479b060f84da6058b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6399175
Commit-Queue: Prasanaa Vingadassamy <prasanaa@google.com>
Reviewed-by: Tommy Martino <tmartino@chromium.org>
Reviewed-by: Adam Arcaro <adamta@google.com>
Cr-Commit-Position: refs/heads/main@{#1439617}