usb-serial-for-android is a driver library for communication with Arduinos and other USB serial hardware on Android, using the Android USB Host API available on Android 3.1+. No root access, ADK, or special kernel drivers are required; all drivers are implemented in Java. You get a raw serial port with Read(), Write(), and other basic functions for use with your own protocols.
$ dotnet add package LusoVU.XamarinUsbSerialForAndroidNo README available.