Zen: Implement calendar event system condition provider.

 - Wire up basic implementation for the system provider that
   handles event-based DND subscriptions.
 - Backed by the standard system calendar content provider.
 - Move shared time utils to base class, clean up logging.

Bug: 20064962
Change-Id: I070b6baa580c592c2ab4101c6b44a254787f9dd7