Default [Q] Where do I find current/new code when installing an apk?

So here is what i'm trying to do.

I use to have a Galaxy Nexus with AOKP, OmniRom, etc installed as well as I have OmniRom on my 2013 N7. My current phone is a Moto X & I have the stock rom on it with xposed installed.

What I like abut Omni on my N7 is that when I go to install an apk, I get the current version installed as well as the new version installed under version info (which you can't find on stock roms). (look at the screen shot attached)

I've been interested in trying to make an xposed module to re-create this for stock roms however I have no idea where to find the string of code that inserts the current/new info you see in the screen shot. I've tried Stackoverflow, but haven't found my answer yet.

It's obviously seen in many custom roms & i'm sure many of you have seen it, but I don't know what portion of the rom it gets coded in, to be able to see what the strings look like to make it work.

Can anyone point me in the right direction? Do I need to ask a rom dev or try to ask this on stack?
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Tmo Moto X 4.4 Xposed