Move source of truth for crypto-related settings to SyncUserSettings

With this change, SyncUserSettingsImpl doesn't need to know about
ProfileSyncService anymore, thus resolving a circular dependency.

Also, most public methods around encryption can now be removed from
SyncService, but that's left as a followup.

Bug: 884159
Change-Id: Ie7aa397ae2e9467a43372061f5199e62d922a7a1
Reviewed-on: https://chromium-review.googlesource.com/c/1414941
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623637}
3 files changed