Add identifer to NotificationResponse.payload so alice's notification can be filtered in notification listener.

[abirajabi]

This PR adds a string to NotificationResponse.payload that sent to the apps that utilize alice package. This string is useful when we have our own onTapped/onClicked listener for the foreground notification.

You can see the complete issue here.

I propose this implementation so that later I can implement this filtering inside my foreground notification listener.

_notificationsPlugin.initialize(initializationSettings,
        onDidReceiveNotificationResponse: (payload) async {
      if (payload.payload == "alice-inspector" && !kReleaseMode) {
        alice.showInspector();
      } else {
        // my custom notification listener goes here
      }
    });

At first I just want to filter the alice notification by using if (payload.payload.isEmpty && !kReleaseMode) since alice sent an empty string to NotificationResponse.payload. However, later I think that it's a bad idea to do so, since other native may also sent an empty string as payload.