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

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https://hg.mozilla.org/integration/gaia-central/rev/173e8cda55a8
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #10999 from mpizza/Bug_893602_HD_camera_master

Bug 893602 - hdpi/xhdpi resolution @1.5 assets for Camera app, r=rex

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https://hg.mozilla.org/integration/gaia-central/rev/4a7871483a3c
Author: mpizza <mmm198219@gmail.com>
Desc: Bug 893602 - hdpi/xhdpi resolution @1.5 assets for Camera app