[unified-consent] Refactor migration code and add settings updates

The migration code in the UnifiedConsentService is refactored:
 - The migration state IN_PROGRESS_SHOULD_SHOW_CONSENT_BUMP is
 extracted to it's own pref kShouldShowUnifiedConsentBump.
 - The migration state kInProgressWaitForSyncInit is introduced
 to be able to update the settings for migration when sync is
 initialized.

Additional changes:
 - ShouldShowConsentBump only returns true if the consent bump
 feature is enabled.
 - The rollback enables sync-everything now also when the user is
 not syncing USER_EVENTS (which is disabled during the migration).

Users that previously had the unified consent feature enabled and
for which ShouldShowConsentBump=true, will not be shown the consent
bump anymore after this CL is landed. This will only affect a few
users on Canary and Dev.

Bug: 863932
Change-Id: I211805f9059cd26056ed81d01ff19470b8caed3c
Reviewed-on: https://chromium-review.googlesource.com/1172690
Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Thomas Tangl <tangltom@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584341}
6 files changed