Zen: Downtime now a condition provider, persist conditions.

- Persist the entire exit condition instead of only the id.
- Make downtime a proper condition provider (similar to the
  existing countdown provider for time-based conditions)
- Move all downtime-related items out of ZenModeHelper and
  into the new condition provider.
- Reevaluate downtime more often, when any of its inputs change.
- Make sure downtime appears as an available condition in the
  condition panel when applicable.

Bug:16296125
Bug:16211189
Bug:17031767
Change-Id: I1d8269a4e6fe170ce776bf932dbbdfb29dd25dd7