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

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https://hg.mozilla.org/integration/gaia-central/rev/57c407d3e94b
Author: Luke Chang <lchang@mozilla.com>
Desc: Merge pull request #17684 from luke-chang/976413_start_stop_activity_window_factory_v2

Bug 976413 - [System2] Add start/stop methods to ActivityWindowFactory, r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/81ffcfd3f088
Author: Luke Chang <lchang@mozilla.com>
Desc: Bug 976413 - [System2] Add start/stop methods to ActivityWindowFactory
Bumping manifests a=b2g-bump