On 2016/12/14 an article was published in MMD: "Wireless MIDI Using
Android Phone": https://www.mmdigest.com/archives/Digests/201612/2016.12.14.01.html 

Although not Bluetooth it is a simple, not very expensive way of
sending wireless MIDI from an Android phone.  It has worked reliably
since it was installed.

Phil Dayson