How to extend Unity Application?

Urban Airship requires me to extend Application instead of (as usual) Activity. Is there any way to do that in Unity?

I tried extending Activity instead but it works only partially. It works and I get notifications on my phone but only when the app is actually running (should work too if it’s not) and when I tap the notification it crashes my app. As I’ve read here: Re-launching android application when user clicks Urban Airship notification - Stack Overflow the problem was that I extended the Activity instead of Application. If extending Application is impossible in Unity is there any way to work around this problem?

Duplicate, it seems: