Recommendation System to integrate with an android app
I don't want to sound harsh but building a recommender system on a android isn't really Data science.

In any recommender system context, the science on data is usually done in ad-hoc manner. 

As for the algorithm implementation, validation and scaling, data science can play a part in that. But this is still not related to the fact of serving the data in a mobile application nor a website. That's mainly computer engineering.

Recommender systems algorithms a quite expensive to compute on any mobile nevertheless. It's best fit to be done by a dedicated server which can also play a role in serving the information when needed. I can't get into the details of this thought as it's quite broad to fit in a single answer on the site.

I advice you to read my answer on the time/space complexity challenge in building this types of application. That may help you getting some insights about the architectural design of a recommender engine in the real world.

I hope that this answers your question.