Bumping gaia.json for 4 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/3a73d93fb722
Author: vingtetun <21@vingtetun.org>
Desc: Merge pull request #23194 from vingtetun/reduce.overpaint.activity

Bug 1055514 - Reduce overpaint when there is an activity covering the cu...

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https://hg.mozilla.org/integration/gaia-central/rev/95290a857eda
Author: Vivien Nicolas <vnicolas@mozilla.com>
Desc: Bug 1055514 - Reduce overpaint when there is an activity covering the current app. r=etienne

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https://hg.mozilla.org/integration/gaia-central/rev/9ae7855cf5b3
Author: vingtetun <21@vingtetun.org>
Desc: Merge pull request #23193 from vingtetun/statusbar-background-dead-code

Bug 1056046 - Remove statusbar-background dead code. r=kgrandon

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https://hg.mozilla.org/integration/gaia-central/rev/6c907e207661
Author: Vivien Nicolas <vnicolas@mozilla.com>
Desc: Bug 1056046 - Remove statusbar-background dead code. r=kgrandon
Bumping manifests a=b2g-bump