Show correct last notification time for grouped notifications

The time value in the current group notifications is not synced with its child notifications.
That causes sub-optimal user experience for time critical contents. We decided to solve this problem with a small group coordinator(GroupTimeCoordinator):

- if all notifications before now, we show the closest time value by now aka maximum value.
- if all notifications after now, we show the closest time value by now aka minimum value.
- if there are notifications before and after now, we show the closest the future time by now. Because, any future event is more important than even the most recent past event.

Test: atest GroupWhenCoordinatorTest
Bug: 181790059
Change-Id: I1bbbffb8d5c7a056f19b743b2984f86611978610