[firebase-messaging] Missing 'foreground' property in onMessage callback on Android

In many instances it is desirable to handle notifications differently depending on if the message was received while the app is in the foreground vs when it is in the background. There is currently no way to determine this right now.

The old Firebase plugin by Eddy had a 'foreground' boolean property to accomplish this. Then in the 'onMessage' callback you could decide how to handle it.

https://github.com/EddyVerbruggen/nativescript-plugin-firebase/blob/master/docs/MESSAGING.md

まず、現在の Android の onMessage callback の動作を、リンク先の Eddy Firebase messaging ドキュメントと比較し、その後、callback payload がどこで組み立てられているかを追跡します。callback が foreground と background のメッセージ配信を区別する foreground boolean を公開し、動作がドキュメント化された例と一致すれば完了です。