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
I would suggest to define the functions under the `VolumeManager` to avoid redundant definition while calling `_sliderHandler` for each channel type.