Reset PrefChangeRegistrar on user session change or start

- The PrefChangeRegistrar::Init method was being called a second
  time when a managed user was added to an existing session,
  leading to a DCHECK failure. This CL fixes the issue by
  resetting the PrefChangeRegistrar (i.e., clearing its observers)
  anytime OnUserSessionStarted is called, ensuring Init can be
  safely called afterward.

Bug: 444423296
Change-Id: Ic82309a41a693fed1e767b76fef2f520508e1e0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6986073
Reviewed-by: Wenyu Zhang <zhangwenyu@google.com>
Commit-Queue: Michael Checo <michaelcheco@google.com>
Cr-Commit-Position: refs/heads/main@{#1522894}