This isn't an important enough specialty to make a living out of, because the number of people who are about what the actual code is such a tiny % of the viewing audience that it's not going to have any meaningful impact on sales. If there's a big budget with lots of VFX (like
Iron Man) the VFX crew will get the job of sourcing 'teh codez' and most likely will just use some deprecated code of their own since high-end VFX software is all script-based anyway.
For non high-end productions (eg where you just have one person with After Effects or suchlike) you can just buy/rent/pirate sample material or fake it up quickly with some 'computerish' looking font - hence the proliferation of fake smartphone screens. It's a pain to photograph a real smartphone with an actual camera; they're usually too bright (even on minimum setting), the screen has reflections, and there are often awkward interference artifacts between the camera's frame rate and the refresh rate & lag of a phone. So if you're doing things on the cheap it's usually easier to just use some color app and make the whole screen a dim green and then match in a generic screen showing a text message or picture of the character who is supposed to be calling.
While this is certainly something a video graphics person could develop a specialty in, you'd have a hard time making it your only thing, same as being a sound designer that only does automobile noises or something. Likewise, when you look at a shot of someone fixing a car you probably don't pause to eyeball the diameter of the nuts and bolts lying on the ground, though pro car mechanics probably hate watching such scenes.