Is it possible to modify the Android Things system image to expose the line in as inputs?. ANSWER. No,
Google will host and update the image of the board that has been tested and approved, developers only can use the USER-SPACE drivers, provided by Google Services.
Can someone confirm that the line in jack supports stereo audio? This is not entirely clear from the datasheet.
Is it also possible to access the line in with the Android Things framework?
ANSWER. It is better to ask ARGON regarding their HW, but based on what it is in the Web Page and in the User Manual, I would said it is possible and Android Things should have the support to handle the Line In using the Audio Service in the user space driver.
Have a nice day sir!