Zen: New behavior for built-in downtime + nextalarm conditions.

 - Downtime: Allow user to enter downtime early, offer as an end
   condition four hours before downtime starts.  Available in
   either none or priority, regardless of settings configuration.
 - Downtime: Always exit before next alarm if zen=none.
 - Downtime: Make more like any other condition provider, remove
   special status (mostly).
 - Downtime: New auto-triggering rules, allow triggering after a
   manual condition ends, once.
 - Decouple NextAlarm + Downtime providers, allow them to offer
   their conditions at the same time.
 - Downtime/NextAlarm: Update conditions if they change while being
   requested, even if unsubscribed.
 - Make all three built-in condition providers optional, via config.
 - New internal helper for runtime config.
 - Don't follow changes to next alarm, consider the condition false.
 - Isolate downtime calendar logic into separate class (for testing).
 - Allow a:bb -> a:bb as a valid downtime range (all day).
 - Volume dialog: configuration establishes maximum number of visible
   conditions, including built-ins.
 - Zen mode panel: avoid widget updates during layout transition.
 - Zen mode panel: move controller callers to background thread.
 - Zen mode panel: hide/show/rebind rows instead of adding/removing.
 - ZenLog: Add downtime autotrigger results.
 - Volume panel: Smarter refresh on ringer/zen changes.

Bug: 16373455

Change-Id: I4f801018ddb0beb6eb9fa03a81c79f7949888a3f