As far as I knew, if you "Force stop" an application, then all the services and processes started from that app are terminated. That's why I didn't try to find a way to keep a message service running after my app was brutally closed, I thought it was impossible.
Then I've found out that if I force close Whatsapp and someone sends me a message, I still get a notification.
How is it possible ?
OK, my fail, it's definetely not possible as designed by Google. I did the test again, force closed whatsapp and didn't get any notification.
So, eventhough pressing a button "Force stop" dosen't offer many ways to fail, I must have done something wrong the first time I tested.