Log out if user is not allowed after settings are changed

1. This is crucial for the OffHours policy implementation,
   as we need to sign out disallowed users when the OffHours mode
   switches off.
2. This change has a side effect of forcing sign-out when the session
   availability (for instance, the Guest mode being allowed via policy)
   changes to false during that same session.

Logic is similar to function here:
https://cs.chromium.org/chromium/src/chrome/browser/chromeos/login/existing_user_controller.cc?l=350&rcl=6fcdeac565292994e832db25b600331dfb030672

Bug: 739713
Change-Id: Ieac2e7f24a152f323b7abb4bf36242a9fef6edc2
Reviewed-on: https://chromium-review.googlesource.com/704662
Commit-Queue: Daria Iakovleva <[email protected]>
Reviewed-by: Sergey Poromov <[email protected]>
Reviewed-by: Maksim Ivanov <[email protected]>
Reviewed-by: Xiyuan Xia <[email protected]>
Cr-Commit-Position: refs/heads/master@{#508075}
14 files changed