First, almost all Android apps are coded in Java. But this is Java for heap sizes in the 10s of MB. Java with a component model that aggressively claws-back memory. Java with tasks that can wake up with a new PID. Java with RPCs you actually would use. Java with a rich UX stack. Server Java brains have evolved in a different direction.

Secondly, Google App Engine seems like a huge convenience for stuff that needs to scale and teams that don't have time to spend on the scaling problem. I don't hear much about it. I wonder why.