Bumping gaia.json for 2 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-2_0/rev/f4936c668cc4
Author: Michael Henretty <michael.henretty@gmail.com>
Desc: Merge pull request #22227 from davidflanagan/bug1041710-v2.0

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

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https://hg.mozilla.org/integration/gaia-2_0/rev/7be18f03a690
Author: David Flanagan <dflanagan@mozilla.com>
Desc: 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
Bumping manifests a=b2g-bump