[OS Notifications] Daily reminders & tips

**User retention feature**

Add notification types to encourage daily engagement:

- Log symptoms
- Check "Exposures" app to stay in the know re: exposures
- Contact tracing: if you receive a call from contact tracer, it's important to pick up the phone
- Reminder that you can prepare for an interview if you are exposed to COVID-19 (different than prompt sent by healthcare official, this does not assume user has been exposed)

No files, tests, or entry points are named. Start by locating the mobile app's existing notification types and scheduling flow, then clarify the timing, wording, eligibility, and platform behavior for each of the four reminders. Done means all four notification types are implemented with verified delivery behavior.

Written by the indexing model from the issue text.