[android] Cannot set custom notification icon

1. What version of RN and react-native-fcm are you running?
11.2.0
2. What device are you using? (e.g iOS9 emulator, Android 6 device)?
Android 8.0 (OnePlus 5)
3. Is your app running in foreground, background or not running?
background / not running

I am trying to change the notification icon. But so far I have not been able to do that. I have read tons of articles about this, but I have no idea what's wrong with my setup. Everything else works as expected.

`AndroidManifest.xml`
```xml

```

The colour is actually working and I see the notification icons tinted:


But they do not have the expected shape. I tried several things.

* I tried to copy the icons from the firebase example project.
* I created custom icons and added them to mipmap and drawables

None of that worked so far. If anyone has an idea what might be wrong, I would be very happy for any hint / help. What's really weird is that android-shaped icon with the droid. I have no idea where that is coming from.

Icon sizes are according to those guides:
* https://developer.android.com/guide/practices/ui_guidelines/icon_design_status_bar.html
* http://iconhandbook.co.uk/reference/chart/android/