Zen: Create a new exit condition for "next alarm".

- If the user's next alarm is in the next 12 hrs, provide
this as an exit condition trigger for leaving none/priority.
- Don't display the next alarm condition when downtime is active.
- When the next-alarm exit condition is active, follow changes
to the next alarm, assuming it remains within the 12-hr window.
- Tweak the downtime condition strings to be consistent.

Bug: 16373455
Change-Id: I4020b91d323dead998e62d655132eca07438b148