Best practices to create an android module

I have an android application and I want to create a piece of code that could be used by that specific application or another.
So that would be either a library or a module.
If I understand correctly a library would be plain Java code while a module would be something that would be using Android APIs.
My question is what are the best practices to create such a module. E.g. is there a problem adding an activity as part of the module?