[Autofill Assistant] After disabling the AA switch in the settings, onboarding must be shown again.

The current intended way the onboarding is supposed to work:
1) The user must see and agree to the ToS before AA starts
2) Initially, the AA switch is off, which will trigger the onboarding screen
3) When the onboarding is shown:
  4) if the user disagrees, onboarding will be shown again (switch stays off)
  5) if the user agrees, the AA switch is turned on and onboarding will not be shown the next time
6) If the user disables the AA switch, onboarding will be shown the next time (goto 3)
7) If the user disables and then enables the AA switch, onboarding will be shown only if the ToS have not been agreed to previously

Bug: 806868
Change-Id: I3e5fee33aa83a3e2ce9c080c29a660319f69a1c5
Reviewed-on: https://chromium-review.googlesource.com/c/1377736
Commit-Queue: Clemens Arbesser <arbesser@google.com>
Reviewed-by: Mathias Carlen <mcarlen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616661}
2 files changed