[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

首先将当前 Android onMessage callback 的行为与链接的 Eddy Firebase messaging 文档进行比较，然后追踪 callback payload 的组装位置。当 callback 暴露一个 foreground 布尔值，能够区分前台和后台消息传递，且行为与文档中的示例一致时，即视为完成。