Bug 1041710: ensure that wallpaper size always exactly matches screen size

when computing screen size for wallpaper pick activity use Math.ceil to get 854 instead of 853.5 for FWVGA

Unit tests for wallpaper_manager.js plus fix a broken bootstrap test

address review comments

address review comments

remove special low-tolerance case for PNG image comparison in image_utils unit test because it caused failures on TBPL
Merge pull request #22089 from davidflanagan/bug1041710

Bug 1041710: Ensure that wallpaper images always exactly match the size of the screen r=mhenretty, timdream