This app was mentioned in 2 comments, with an average of 2.00 upvotes
Check out Brainwave... It allows you to skip and stop songs using hand gestures over the phone even when the screen is off
https://play.google.com/store/apps/details?id=io.onthego.apps.brainwave
BrainWave. Works on hand gestures using the front cam. https://play.google.com/store/apps/details?id=io.onthego.apps.brainwave