Bug 973445 - [settings] refactor Sound panel with AMD pattern

Note: this refactor just move slider control into `volume.js`

Need to make sure https://github.com/mozilla-b2g/gaia/commits/master/apps/settings/js/sound.js are not updated from 8/29 as well
This assumes that the order of the slider elements are the same as `channelTypes`, which is not robust. Please pass an object explicitly specifies the slider element for each channel type.