Update MLKit dependency + Restructure code

• Update MLKit dependency, it's no longer part of Firebase.
• Restructure and simply code. Unnecessary classes were removed, and FaceBoundsOverlay was simplified so that it renders any boxes it receives. The heavy-lifting (which includes correcting the face bounds, scaling, and mirroring them) was moved to FaceDetector.
• Improve face detection. The previous face bounds correction was not entirely correct, which was resulting in the face bounds not being rendered exactly around the face (there were often offsets).