[iOS] Migrate PasswordManagerViewController to IdentityManager

PasswordManagerViewController was observing `identityListChanged`
from ChromeAccountManagerServiceObserver, for the purpose (per the
CL that originally introduced it, crrev.com/c/1856078) of reloading
itself on signout.
First, this doesn't necessarily work, since a signout doesn't always
come with a change to the identity list.
Second, it'll cause a reload in many cases where it's not necessary.

This CL changes PasswordManagerViewController to observe
IdentityManager instead, specifically `onPrimaryAccountChanged`
which matches the intention much better.

Bug: 377467350
Change-Id: I0010a6a519e8698ca6ecfb712eec9dce414a202d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5999570


Reviewed-by: Viktor Semeniuk <vsemeniuk@google.com>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1378973}