This week Google released developer preview 3 of Android Things. In it they added support for USB-Serial UARTS. I have previously shown how to add USB-Serial ports to android things using the NDK. It seems this is no longer necessary.
With developer preview 3 the list of available UARTs returned by getUartDeviceList() will include connected USB-serial devices. My QUAD UART from FTDI was detected by the kernel but the peripheral manager culled all but one of the UARTs as described in this issue. Single FTDI devices seem to work OK.