Handle Firebase foreground notifications

[epou]

See: https://firebase.google.com/docs/cloud-messaging/flutter/receive#foreground_and_notification_messages

For Android: set high priority (backend -> Mosquito-Alert/mosquito_alert#293)
For iOS: check method setForegroundNotificationPresentationOptions

Example: https://github.com/firebase/flutterfire/blob/6c0eb6c7f4415838b8ebcbf796499dd2ef59da43/packages/firebase_messaging/firebase_messaging/example/lib/main.dart#L87