[firebase_messaging] onMessage and onBackgroundHandler are run on main thread in android

[orkunsoylu]

The Issue
I'm using the data messages to get a payload from the server and using the data to do network operations. In native platform, this is not creating any problem since FirebaseMessagingService is not run on main thread. However with your plugin when I use the same structure, android.os.NetworkOnMainThreadException.

Expected behavior
The callbacks should run on an another thread then the main thread, just like the behavior in FirebaseMessagingService