Bug 830644 - Support WVGA and other non-HiDPI screen dimensions and resolutions

* system [statusbar, quick settings, utility tray, notifications, sound manager]: px to rem, added background size definitions, refactorized sound manager for easy device adaptation (no js changes required).
* added @2x assests for system [statusbar, quick settings, utility tray, sound manager]

@timdream @rexboy7 @jmcanterafonseca @vingtetun  take a look!