I miss the Jellybean ability to adjust things such as brightness and contrast with the KitKat update.
Is there an api that will bring back those abilities, like a whole new camera app, or the old one?
You can't really adjust brightness and contrast in a camera, those are display settings. "Adjusting" them in a camera is a gimmick - it doesn't really adjust contrast and brightness.
Get something like Camera FV-5 Lite for now, and read what Kodak says (they have hundreds of different tutorials on line) about adjusting the contrast and brightness of a picture when you're taking it.
Then wait for the Lollipop camera app to become available. It can do whatever your camera and its firmware are capable of, as far as adjusting settings. (If the camera can't do it - say variable F-stops - no software can.)