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

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https://hg.mozilla.org/integration/gaia-central/rev/d4286405bf5c
Author: Ran Ben Aharon <ran@everything.me>
Desc: Merge pull request #20893 from EverythingMe/1028691-bg-quality

Bug 1028691 - [Collection app] Background images sometimes in poor quality

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https://hg.mozilla.org/integration/gaia-central/rev/15d727b89092
Author: Ran Ben Aharon <ran@ranbena.com>
Desc: Bug 1028691 - [Collection app] Background images sometimes in poor quality

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https://hg.mozilla.org/integration/gaia-central/rev/cb1b38843af5
Author: Florin Strugariu <bebef_1987@yahoo.com>
Desc: Merge pull request #20982 from zacc/bug_1026424

Bug 1026424 - Remove explicit frame switching where activityWindow is us...

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https://hg.mozilla.org/integration/gaia-central/rev/e8b52d0ba369
Author: Zac <zcampbell@mozilla.com>
Desc: Bug 1026424 - Remove explicit frame switching where activityWindow is used
Bumping manifests a=b2g-bump