What's a good unit testing assignment for Android?
I assume that there is some logic in the app, somewhere. Hard for me to imagine it otherwise. 

If you build the client side with a Model-View-Controller architecture, the testing can be done on the model (as is most natural) and possibly on the controller. 

If the server side is also built by the students, then it may also require logic beyond data access. So that can also be tested, either directly on the server side, or with mock clients. Maybe both. 

I admit, though, that I don't build these things.