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

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https://hg.mozilla.org/integration/gaia-central/rev/a7b196a19c49
Author: cctuan <georgeiscoming@gmail.com>
Desc: Revert "Bug 922462 - Define various size of icons in manifest instead of use buildscript"

This reverts commit 32153515a3eedad26aec89322e02934a0dc06a0f.