MIDI Keyboard
with Android API 24

select Reciever Port in spinner

Example 1
send MIDI message from MIDI Keyboard to MIDI Synth inside Android

STEP 1 : run MIDI Keyboard app
STEP 2 : select Reciver Port of MIDI Synth
STEP 3 : MIDI sounds , when touch Keyboard

Example 2
send MIDI message from MIDI Keyboard on Android to MIDI Synth on MAC

STEP 1 : connect USB cable MAC to Android device
STEP 2 : select MIDI in USB mode on Android

STEP 3 : select USB port in MIDI Keyboard on Android

STEP 4 : run MIDI Synth on MAC
STEP 5 : MIDI sounds from MAC, when touch Keyboard on Android

• https://developer.android.com/reference/android/media/midi/package-summary
  
• https://github.com/philburk/android-midisuite/tree/master/MidiKeyboard