Looking into making a embedded device that uses usb with an android phone and so looking at AOAP and ADK etc and most, if not all, I can find about the embedded devices is in/about Arduino. Does this mean that the embedded device has to be an arduino? Or can another device such as an Atmel board using C be used too?
You can use any kind of micro controllers or CPUs as long as it supports Usb or you can use a ftdi adapter to convert from UART or I2C or SPI ..... to USB. Arduion has an atmel micro controller with the ft232 chip built into it.