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

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https://hg.mozilla.org/integration/gaia-central/rev/c243aae1ad25
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Merge pull request #14039 from etiennesegonzac/bug-942156-better-sheet-transition

Bug 942156 - Refining the app to app sheet transition. r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/319e62c6b5d5
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Bug 942156 - Refining the app to app sheet transition.

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https://hg.mozilla.org/integration/gaia-central/rev/7e6784c0a594
Author: Cristian Rodriguez <crdlc@tid.es>
Desc: Merge pull request #15259 from crdlc/bug-949411

Bug 949411 - [Download Manager] The download list is emptied after closi...

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https://hg.mozilla.org/integration/gaia-central/rev/424714086a16
Author: crdlc <crdlc@tid.es>
Desc: Bug 949411 - [Download Manager] The download list is emptied after closing the settings app