I have a wireless MIDI solution for the Android but it does not simply
use the Bluetooth capability in the Android device -- it requires a
combination of products.

(1) The Android app is "MIDI Voyager Pro" by "bytebolt" from the Google
Play store.  It is a MIDI player that will direct the output to the USB
port.  It is a very nice app and well worth the nominal fee for the
"Pro" version, which is required for the MIDI out capability.

(2) The next product is the iRig MIDI 2.  iRig makes a cable that will
go from the micro USB port on the Android device to their unit which
then has a full set of standard MIDI cable ports: In, Out and Thru.

(3) The final product is the midiBeam from panda-audio.  It plugs into
the iRig MIDI 2 and transmits to the receiver which is plugged into the
device you want to play, in my case a Chickering Ampico.  All of that
lashes together in a unit that is 2" x 3" x 1-3/4" and plugs into the
Android device.

The midiBeam receiver is fastened underneath the piano.  (I have used
both a Samsung Galaxy S tablet and a Samsung Note 5 cell phone to play
the piano.)  I have the midiBeam receiver plugged into a smart power
switch, so to play the piano I just plug the unit into the Android
device, hit the icon to turn on power to the receiver, flip the slide
switch on the transmitter, which pairs automatically with the receiver,
then tap the app and go.

I think if you could make your own cable to go from micro USB to a
female MIDI receptacle, you could probably eliminate the iRig product,
but haven't tried it and did not find such a cable available for
purchase.  (The midiBeam transmitter is very small, about 2-1/2" x
1-7/8" x 1/2".)

An alternative approach is to use a Samsung utility that allows one
of their tablets to mirror the screen and control one of their phones.
In that mode, I have plugged the phone into the iRig and left the phone
simply plugged into the piano (through the iRig MIDI 2).

Then from the tablet, with nothing else plugged into it, I can run the
phone just as if I were holding it.  That eliminates the cable into the
tablet, but I think it is more cumbersome overall.  It is dependent on
the phone battery and you have to take several steps to establish the
pairing between the phone and tablet.

Hope this helps.

Deane Shepard