Use APKPure App
Get Midi Host old version APK for Android
Software synth for USB Midi keyboards
Plug in a USB Midi keyboard and play it. No settings, no distractions.
Supports legacy devices that the Marshmallow Midi API doesn't. All the heavy lifting is done by two great libraries, https://github.com/kshoji/USB-MIDI-Driver for communication with the device and https://github.com/billthefarmer/mididriver for sound generation.
Source code is available at https://github.com/bndeff/midihost