This is a test app intended for software developers and OEMs to test Android MIDI on Marshmallow. The sound is not fancy. Just a sawtooth wave and an envelope.
You can connect a MIDI keyboard by USB or Bluetooth and play notes. Or drive the synthesizer from another MIDI app such as the Mobileer MidiKeyboard. Connect to the "Mobileer, SynthExample" port.
Complete source code is available on github under "philburk/android-midisuite".