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
(cherry picked from commit 7815e901b5ccec40b22987a10d002bb1225d8d17)

Conflicts:

	apps/settings/js/panels/display/wallpaper.js

remove special low-tolerance case for PNG image comparison in image_utils unit test because it caused failures on TBPL