Show chronometer in expired timer notification

Bug: 28985277

The chronometer time is always displayed as the top line.
The second line may now be any of:
- a single timer label
- "Time's up" if no label was given
- "N timers expired" if multiple timers are currently expired.

Change-Id: Ibe57b03b4b308d89f11ba0d9f175d16bee2480eb