Midi Recorder is a very simple Android app that records data from a MIDI device and saves it to a file on the Android device.
Note that this app simply acts as a UI and does not actually communicate with the MIDI device, but rather with an intermediate server. The Midi Recorder server must be running on a computer that is directly connected to the MIDI device.
The APK must be installed on your Android device manually. You can download the latest version of the app here. You can find setup instructions for the server here.
Why use this app and have to bother with cables and servers instead of recording over MIDI bluetooth? I have yet to find a decent Android app that produces MIDI files, or even acceptable quality audio files. I found recording over a wired MIDI connection produced much better results, though I don't have space next to my keyboard for my laptop. My solution to this problem was this app. If you have this same problem then hopefully this app will help you too!