Zero.
Or maybe just the semantics of Java and basics of OOPS (to be honest).
You don’t need to learn a particular language as a whole to be a good android app developer. Perhaps you need to be adept at googling, using stack overflow and reading and understanding the official documentation.
The most common language used for android app development is Java therefore I am writing this answer in the context of Java. What I feel is that just a basic knowledge of Object Oriented Principles and a feel of objects and classes will suffice. I don’t think any developer is aware of detailed working of any API for android (except the ones who have developed it :P ). Perhaps you don’t need that too.
The most important criteria to be a successful app developer is having a good idea in mind for the app and the features to be implemented in the app. There are official docs available which can be comprehended via thorough reading of the code and its explanation....