Fix an issue where internal Calendar DateTimes would not be correctly set to all-day

Summary:
Ref T11816. I don't really know what happened here, maybe I rewrote and broke this at the last second?

In most cases, we directly respect the isAllDay flag on the event, so the internal date state doesn't matter too much.

However, in the case of mail notifications, the raw internal state is relevant. This should fix mail notifications for all-day events.

(I might still turn them off since I'm not sure they're too useful, but it's good to have them working.)

Test Plan:

• Created a new all-day event, verified database values wrote correctly.
• Ran bin/calendar notify --trace, verified it picked up an all-day event tomorrow with a large enough --minutes value.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11816

Differential Revision: https://secure.phabricator.com/D16954