Bumping gaia.json for 1 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-2_0/rev/a2ae86cab720
Author: Zac <zcampbell@mozilla.com>
Desc: Bug 1021767 - Remove switch_to_displayed_app from _set_pref/_get_pref for consistency
Bumping manifests a=b2g-bump