It sounds like your question is, how do you go from a UART device to something that can be plugged into a microphone jack.

http://electronics.stackexchange.com/questions/12498/what-protocol-does-a-headphone-jack-use/12504#12504 should help you understand what is being done on the existing device that you are talking about.

There is still a link missing for you though, that is the ability of turning UART into the audio itself. The easiest way to do this is to buy a microcontroller that you will place between the UART device and the Android device. All the microcontroller will do is read in any UART data and then convert it to what ever form you want the headphone jack to receive. You may also need to implement some handshaking between the microcontroller and the phone in order to know that the device you think is plugged in.