A mobile app wizard, when finished have an additional function (if used - screen isn't changed).
Does it should have: 1. A separate "Back" (to main screen) button or 2. it's enough allowing the user use system's built-in "back"?
"Back" means that you go back to the previous screen. The android back button also serves the same principle.
Since your app's back button would bring the user back to the home screen rather than back to the previous screen, I'd call the button differently. Maybe "Done" or "Home".