I'm very experienced on Unity and iOS and I can deploy my Unity app to Android. But I don't know anything about native Android stuff.
My problem is about receiving a GCM message and showing a notification when the android app is in background. I implemented it and it works perfect when the app is in foreground but my GCM message listener isn't being triggered when the app is in background and I can't show the notification.
I use this plugin for receiving GCM messages : https://github.com/kskkbys/unity-gcm I use this for showing the notification : https://www.assetstore.unity3d.com/en/#!/content/9484
I tried to reach out the authors of these plugins but they didn't reply for a long time...