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

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https://hg.mozilla.org/integration/gaia-central/rev/4353f52bcb15
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Merge pull request #14165 from etiennesegonzac/bug-943246-call-screen-background-tweaks

Bug 943246 - Smarter call screen background handling to prevent flashes. r=rik

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https://hg.mozilla.org/integration/gaia-central/rev/bf4da9108dc0
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Bug 943246 - Smarter call screen background handling to prevent flashes.
r=rik