I've got a micro USB to OTG adapter to USB-B with which I can connect my Android tablet to a Casio keyboard. On some apps such as WalkBand I can play on the keyboard and the app receives the MIDI events and plays them in the app as well, and can even record them.
However, at this point I want to actually do something with the MIDI notes besides just be able to play them. I want to see them on a piano roll, and maybe even as sheet music, and be able to from the piano roll edit, either to move notes around, adjust the length, snap them to the appropriate 1/4, 1/8, 1/16 etc. (this would be a batch operation) as well as add and remove notes.
Can be free or for pay.