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

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https://hg.mozilla.org/integration/gaia-central/rev/fc54daf499af
Author: John Hu <johu@mozilla.com>
Desc: Merge pull request #18007 from huchengtw-moz/system2/Bug_971554-reland-3

Bug 971554 - [System2] Instantiable SoundManager, r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/0b39d9acab2a
Author: John Hu <johu@mozilla.com>
Desc: Bug 971554 - [System2] Instantiable SoundManager

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https://hg.mozilla.org/integration/gaia-central/rev/4b8ff547d652
Author: Wilson Page <wilsonpage@me.com>
Desc: Merge pull request #19865 from wilsonpage/1008187

Bug 1008187 - [Camera] Update to use <gaia-header>

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https://hg.mozilla.org/integration/gaia-central/rev/10f454ed0a59
Author: Wilson Page <wilsonpage@me.com>
Desc: Bug 1008187 - [Camera] Update to use <gaia-header>
Bumping manifests a=b2g-bump