Bug 896231 - hdpi resolution tweaks update for System app

1. remove bottom position of timebar.
2. iframe.getScreenshot(w,h) use device_ratio*width/height
3. set background-size as 1.5rem for prev.png and back.png 
4. replacing signal-searching.png with gif 
5. update backgroundsize attribute 
6. fix wrong background setting in update_manager.css