Merge ActiveNotification and PendingNotificationRequest?

[Levi-Lesches]

Active notifications have all the fields pending notifications do, plus more optional/nullable ones, so these can naturally be combined into one class, something like PlatformNotification. With typedefs, this change would be invisible to end users and would reduce some code duplication across platforms.

I'd have to check into it more, but it seems this would also allow pending notifications to return more data. Eg, on Android, we control what data is saved via SharedPreferences, so it would be trivial to save more notificationDetails fields