Interface design to have multiple implementations of network call libraries (Retrofit or Volley)
Let me point out an other aspect, you might miss at the moment. I know that this is a learner question/example, but keep in mind that your approach is designing an application where you can switch the network library (in extreme) at runtime. This is not the most common scenario.

Actually it is more likely that you want to switch Retrofit against a mock or Volley against a mock in your tests. So you have done everything right, if you can write tests for related parts of your system (e.g. the StatusFragment)  without doing real network calls and there is no test switch in your production code. (The network stuff will get it's own test.)

On the other hand, although I'm not familiar with the libraries, Google told me that both seems to have some own mocking concepts. So there might not even be a need to switch your implementation in unit tests. In case you finally end up with only one implementation now and optional a total replacement in the future, this approach might add an unnecessary (mental and technical) overhead.

Implementing against an interface don't necessary mean a Java interface. If you only have one implementation at a time, it is totally fine to drop NetworkCallsApi and move NetworkCallsRetrofitImpl into ServiceCalls. Afterwards you are implementing against the interface of ServiceCalls and you don't care about is implementation. It is also pretty straight forward to change the implementation in ServiceCalls later without changing other parts of the application.