[signin] Add SigninManager::Shutdown()

This is a cleanup with no behavior change.
The SigninManager did not implement the Shutdown() method.
However, it is not safe to call the dependent services after shutdown.
This CL adds the Shutdown() method, where the SigninManager unregister
the observers, to prevent any further use of the IdentityManager.

This CL also updates the code to use scoped observations rather than
naked observers.

Change-Id: I608471b646cc8d60b72f51306b051ffb46c1c92e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3436165
Reviewed-by: Jan Krčál <jkrcal@chromium.org>
Commit-Queue: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/main@{#967190}