I have my android app in which a customer can order food so I want to integrate alexa voice service to order food on behalf of customer.
This sounds like a great idea. Do you have a specific query regarding AVS? Otherwise, you could get started on this with an Alexa Skill. A good place to start conceptualizing this would be over here:
Your android app would implement the AVS APIs to call your food ordering skill.
Android App and Amazon Alexa Speaker 1 Answer