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

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https://hg.mozilla.org/integration/gaia-central/rev/0976a3ff6388
Author: Luke Chang <lchang@mozilla.com>
Desc: Merge pull request #16356 from luke-chang/971452_instantiable_activity_window_factory

Bug 971452 - [System2] Instantiable ActivityWindowFactory, r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/a39e5e7ba971
Author: Luke Chang <lchang@mozilla.com>
Desc: Bug 971452 - [System2] Instantiable ActivityWindowFactory
Bumping manifests a=b2g-bump