[AF] Add a wallet sync transport opt-in pref.

This will be used to record whether a user has opted in to seeing their
Google Account cards in the Autofill dropdown.

The dictionary should not contain info about accounts that are not opted
in.

Bug: 907929
Change-Id: Icc663b228792c58b00c02b204249fba9f557481b
Reviewed-on: https://chromium-review.googlesource.com/c/1348792
Reviewed-by: Fabio Tirelo <ftirelo@chromium.org>
Commit-Queue: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610621}