ios notif: Don't show notifications for logged-out accounts

[gnprice]

This is the iOS equivalent of:

• Don't show notifications for logged-out accounts, on Android #1264

This is enabled by #1265, which we'll have completed soon.

Our implementation of E2EE notifications, #1764, should handle this itself for those notifications. For this issue, we should also handle it for notifications from the legacy protocol, which includes all notifications from servers up through 11.x.