Using the RPi GPIO, you can access many communication protocols and use them to transfer raw data :
Serial Communication You can send datas from the RPi with the serial protocol to the android USB port. Transfert rate will be at best around 250kbps.
Here is a sample of implementation : Android side : http://www.instructables.com/id/Arduin-Adroid-USB-Serial-Communication/ RPi side : http://www.instructables.com/id/Read-and-write-from-serial-port-with-Raspberry-Pi/
I2C communication
If your android device has a I2C interface, you can use also use this protocol, wich allows a transfert rate up to 3.2Mbits/s.
As this protocol is not specially designed for data transfer, you may face some unexpected hardware limitation.
Here a sample implementation :
Android side : https://developer.android.com/things/sdk/pio/i2c.html
Rpi side : http://www.raspberry-projects.com/pi/programming-in-python/i2c-programming-in-python/using-the-i2c-interface-2
SPI communication
If your android device has an SPI interface, this may be your best option.
This protocol will work only with short cable in order to enjoy the very high transfer rate, from 25 to 50 Mbps/s.
Here is the sample implementation : Android side : https://developer.android.com/things/sdk/pio/spi.html
RPi side : https://github.com/doceme/py-spidev