Notification Refactor — Examine Current State of Notification Architecture and NotificationSender class

It appears as though we are using a feature flag, UseNotificationSenderForSystemNotifications. This is only set to true currently for debug builds of both K-9 and Thunderbird Mobile. Testing done in debug against release and beta issues (and vice versa) can't be trusted. We also don't seem to have a set migration path. Our notification documentation does not mention that the behavior in the documentation is behind a feature flag that is currently disabled.

For this ticket, see what it would take to implement the new NotificationSender class for all system notifications in debug, then what we'd need to do to move it over to our beta and release builds as well. The output from this ticket should be clear documentation listing:

1. What is in use in notifications for release and beta builds now
2. Where we use the new NotificationSender described in our notification documentation
3. Why we haven't moved to using NotificationSender app-wide yet?
4. What would it take to migrate?
5. Are there opportunities to improve the reliability of notifications through the existing NotificationSender framework for system notifications?