[iOS] ChromeAccountManagerService: rename pref_service_ -> local_state_

The name "pref_service" is typically understood to refer to the
*profile* pref service, but ChromeAccountManagerService actually uses
the *local state* pref service.

While we're here, this also adds a CHECK_IS_TEST() to ensure the pref
service can only be null in tests.

Bug: none
Change-Id: I1e290bc810477c25f3c7fce36082ffb51921b068
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5964924
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1374582}