MessageReceived event doesn't fire when app is in background #877

[REQUIRED] Please fill in the following fields:

• Unity editor version: 2018.4.29f1
• Firebase Unity SDK version: 6.16.x
• Source you installed the SDK: Unity Package Manager
• Problematic Firebase Component: CloudMessaging
• Other Firebase Components in use: RemoteConfig, Analytics
• Additional SDKs you are using: GoogleMobileAds
• Platform you are using the Unity editor on: Mac
• Platform you are targeting: Android
• Scripting Runtime: Mono

[REQUIRED] Please describe the issue here:

FirebaseMessaging.MessageReceived does not fire when I click on notification and the app is opened from the background.
I experience the issue in SDK versions of 6.16.0 and 6.16.1, I see no problem in version 6.15.2.

Steps to reproduce:

1. Open app.
2. Send notification.
3. Notification is received. OK.
4. Hide app (so it stays in the background)
5. Send notification.
6. Click on the notification to open the app.
7. Callbacks registered to FirebaseMessaging.MessageReceived do not fire.

reproducible with the Firebase Unity quickstarts, 100% of the time.