If Android app is a client of your backend service it should have its own layouts/forms etc. Then, after collecting all necessary information from user you can send a response to server using JSON, in example:
POST http://mydomain.com/users/new
{"name":"Anrnold","password":"I<3steroids"}
If you want to display custom form retrieved dinamically from server you can pass form's fields and generate form on android device programatically. But why would you want to do that?
Everyting you need to know about json is here: json.org. I also recomend Gson