Zen: Support external condition providers.

 - Manage provider subscriptions for external condition providers
   within ZenModeConditions.
 - Move "is automatic rule active" check into common location.
 - Add constant for external rule setting configuration page.
 - Consolidate log tags.
 - Reset rule snoozing on boot.

Bug: 20064962
Change-Id: Ida207dbf363f61e1727974e611f43f27c23accfb