If you are talking sending MIDI messages or receiving MIDI messages over a communication channel, we have several options to be starting from.

1) Bluetooth

2) USB-host

3) WiFi

etc.

I have tried developing a MIDI driver using the USB-host capability and it is currently placed along with my open application, XY controller for MIDI.

https://github.com/MIDIeval/MIDIeval/tree/master/Imperi0usB/MIDIeval

You can use the code to send MIDI signals over a USB link. I have successfully triggered Note ON/OFF messages and developed the XY controller to control performance in real time. eg) Sweeping the cut-off or performing fade-in/out.

The problem that I see with it is, Android does not support isochronous transfers over USB, thereby, limiting us in terms of latency of transmission. The USB transfer is happening in Bulk Transfer mode which does not guarantee low latency as it waits for the acknowledgment.