Bug 1017294 - Add fastforward/rewind to music widget

If I recalled correctly, when I was implementing the fast-forward/rewind features, at first I tried to listen to the click event just like you do, but I found if my finger touches the next/previous buttons but move to outside of the buttons, then leaves the panel, the click events will not be triggered so the fast-seeking function will not stop, it will keep seeking and we are unable to stop it.

I would suggest to listen to the touchend events to stop the fast-seeking because it can guarantee the stop fast-seeking function will be called, you can check out music's Player.js and see how we did it:
https://github.com/mozilla-b2g/gaia/blob/master/apps/music/js/Player.js#L977-L995