Having been in this kind of situation before: because the software isn't ready for any kind of release. They probably spent a week gathering and tuning the parameters on the DSP for each individual phone they ran it on, and without their knowledge of the system it'd take you a month and a half to get it working on your phone.
If they are publishing a paper, their methods (including custom source code) are ready for release for peer review. Ftheit software sucks, thet should not rely on it for scientific results.
Here's a case that I have published under before: Their software is ready for their own use for research by them personally. However, it has no help, documentation, automation, magic, or even small amounts of assistance. It doesn't work until you have it spit out a page of numbers, which you feed into Matlab and hack on for a week before compiling a new copy of the app that has the resulting calibration matrix baked into the code. The instruction manual for the apk you're asking for would be equivalent to a bachelor's in computer science and half a PhD on DSP and machine learning. That's why they're not releasing it. It proves that their math works and the approach is valid. Replacing their expertise and making it fast enough and reliable enough for general release would require a startup, six developers three of whom need PhDs, a UX team, and a year and a half of work. The approach works and they can prove it. That's it.
