How can we use a single git repository for Android application + web service back-end ? Generally its done separately . But its mandatory for us to use a single repository and our contribution to be shown .
You could do it in one repo. This could be achieved by just putting everything into one folder or (recommended way) put everything in dedicated, Independent branches (alias orphan branches).
A good explanation with examples is given in this post's solution
To learn more about orphan branches, I recommend this article on bugfactory.io
Edit: Fixed Typo