[MIM2] Fix more integration tests for multi profile

* Initialize UserPolicySigninService when the primary account changes.
* Use signinWithFakeManagedIdentityInPersonalProfile where needed.
* Stop checking for UI elements that are irrelevant for multi profiles.

Bug: 400648043, 379077722

Change-Id: I0f69f66a9ef4f7896599c98cc60e50bab296a964
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6364834
Commit-Queue: Yann Dago <ydago@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1440519}