I'm tacking onto this, as I also want to be able to write wrappers for various SDK's, and I haven't found the example code to follow along perfectly yet. (as in the Game Play Services SDK)
In looking at this piece of code, I believe the issue is the listener?
In the onCreate method, the listener is set to "this", which if I understand correctly is the ExampleActivity reference, so that the hook exists to call the "onInterstialLoaded" piece of code.
I see in your code that you call the onCreate method in the InitializeInterstitial, but don't think that by declaring and using the listener is the right way to get the hook for your code? (please correct me if I'm wrong!, but if I'm wrong, please show how to do it the right way...

)
I am looking forward to Erel's port of the Game Play Services source code, as there are a few listener methods in there, so I can understand this better myself. (and I really, really want to add social integration using that library...)
Ross