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

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https://hg.mozilla.org/integration/gaia-central/rev/ffb53607d294
Author: DouglasSherk <github@sherk.me>
Desc: Revert "bug 1021857 - Ensure z-index for AttentionScreen makes it top-most."

This reverts commit 38a367ba17e8ce3f2a44884c584448f977a81eaf.
Bumping manifests a=b2g-bump