Python or Java as Backend Language in Google App engine?
Here are some points to consider:

Both Python and Java are capable languages and App Engine Services are available to a large extent in both the environments.
You should use the environment that you are most comfortable with. This will help when debugging issues on the Server side. I would go with the language that I am most familiar with in case the application is critical, is on a tight deadline, etc. If you are learning the environment and have the time, it is great to look at a new language. 
Since you are writing an Android application that is interacting with your Server side application in App Engine, one assumes that you would be exposing this functionality over Web Services. Both Python and Java environments are capable of hosting Web Services. In fact, with Google Cloud Endpoints, you should be able to even generate client side bindings (client libraries) for Android that integrate easily.
I am developing an application in Android using Google App engine and Google Compute Engine as backend . 
I have followed the Google's demo code in python as base for my application. 
Now I have question in my mind that since I am more familiar with Java then Python and also need to consider the fact that Google is supporting Python more then Java in its most of the demo codes, Should I change my GAE backend language to Java?? 
I should stick with Python and hope that I would come around with Python eventually. 
Any suggestions are appreciated. Thanks