Split LivenessService out from LibCrosService.

This is part of a larger effort to split LibCrosService into smaller
services for mus+ash.

I've added a TODO to remove the old service once we've migrated
clients over to call the new one.

BUG=692246

Review-Url: https://codereview.chromium.org/2741803005
Cr-Commit-Position: refs/heads/master@{#456411}