[scatterbrained2] suggests that some Android phones will recognize the FT232H chip, which has GPIO as well as the normal serial modes.  

I have a 232H board with some onboard EEPROM, and it can be configured to do fast SPI / I2C / JTAG, passing all that over USB.  This would be a powerful/fast interface if it worked with a phone.  But I think it set me back $20.  More than a Pi Zero, less than a Pi 3B.