While I have been talking around it for a while, I wanted to better publicize our work on GNU Radio for Android. We can now build applications that run GNU Radio flowgraphs on Android (>= 5.0). We build the flowgraph in C++ and link it through to a Java app for the user interface using the JNI. I have tried to collect the howtos for getting all of the various parts up and running here:
Last week, we got UHD support working, and I felt this was a big marker in our progress on the project. To top it off, we now even have ControlPort support on Android apps, which makes it possible to provide remote control and run tools like gr-perf-monitorx, from which I'm already finding out new things about GNU Radio and Android device performance issues.
There's still a big warning about this being a work in progress, and it definitely is. However, with the support for all of the various hardware and tools that we now have, I think we're at a place of usability for a lot of applications.
Feel free to ask questions if things in the wiki don't work for you. I've tried to go through it multiple times and make it as robust as possible and mostly things should be copy-and-paste. But as things change, and they do rapidly here, we're likely to continue to need some evolution of the instructions.
I will hopefully post some of my simple applications online sometime soon, as well.