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

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https://hg.mozilla.org/integration/gaia-central/rev/b9d57c835bed
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Merge pull request #18694 from etiennesegonzac/bug-1000047-callscreen-race

Bug 1000047 - Hardening the callscreen against a transition end race. r=rik

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https://hg.mozilla.org/integration/gaia-central/rev/302e203016ef
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Bug 1000047 - Hardening the callscreen against a transition end race.

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https://hg.mozilla.org/integration/gaia-central/rev/36b5d0978040
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Merge pull request #18580 from etiennesegonzac/bug-997701-tone-player-channel

Bug 997701 - Set TonePlayer's channel when creating the audi element. r=gsvelto, rik

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https://hg.mozilla.org/integration/gaia-central/rev/eac585f29fda
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Bug 997701 - Create a new AudioContext each time the TonePlayer changes channel.
Bumping manifests a=b2g-bump