[Ongoing call] Don't show the time if the notification's `when` value
isn't valid.

Test: manual + new unit tests (OngoingCallControllerTest,
OngoingCallChronometerTest)

Fixes: 192379214
Bug: 183229367

Change-Id: Idc5097de017ec662cf6c54b5afc196343eb0e3dc
4 files changed