For the callback we need to implement the IOnMapReadyCallback interface, which simply has a method which gives us the GoogleMap instance.
As you can see I simply implemented that interface, and as all interfaces coming from the Java world which expects a Dispose() and Handle implementation we need to inherit from Java.Lang.Object. I also added an event to that class instead of passing a reference of my Fragment into that class.
Now you might be using the SetupIfNeeded() method which Xamarin also uses in their samples and is called in OnCreate() and OnResume() and where ever you feel like it. For that purpose I have adapted that to use the callback class.