[Sync::Consent] Split ConsentAuditor interface from implementation.

Also rewrite FakeConsentAuditor to implement the interface instead of
inheriting from the implementation. This unblocks enabling the separate
datatype feature by default.

Bug: 862186
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I474f74f12278221ef6278c120576f1ff1a224c4a
Reviewed-on: https://chromium-review.googlesource.com/1131508
Reviewed-by: Markus Heintz <markusheintz@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: vitaliii <vitaliii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574569}
12 files changed