[Feature Request] FCM build notification when app is in foreground

## What feature would you like to see?
With FCM when the app is in background the OS create a notification that has the image, icon and everything...

but to build a notification that looks the same when the app is in foreground you shouldn't be having to do it manually, I think we should have a method that receive the Notification object and provides the Notification Builder already filled with everything.

For instance I think the OS also download the image (if there is one) and build a bitmap to add to the notification and lot of other stuff.

## How would you use it?
Instead of having to manually support all the things that Firebase console allow with a notification on my foreground app I would like to just have the option to make it "work" the same way of the OS one produce when the app is in background (still intercepting, but having an easy way to produce the same result).

首先检查 Firebase Android SDK 对前台和后台应用的 FCM 通知处理。确定通知 payload 在哪里进行处理，以及所请求的 builder API 应如何接入，然后将完成标准定义为：生成具有与后台通知相同的受支持内容的前台通知。

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