Change functionality of Navigation bar in Android M

I want to edit the functionality of my Navigation bar. I would like to change the "app switcher button" to "Switch to Last App" and long-press to be "app switcher button". I know this is doable with Xposed Framework and modules, but I am running stock Marshmallow on my Nexus, so that's not an option. I believe I can edit System UI to do this, but not sure how or where to go from there.