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

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https://hg.mozilla.org/integration/gaia-central/rev/ac08c443a9f4
Author: Eitan Isaacson <eitan@monotonous.org>
Desc: Merge pull request #18699 from yzen/bug-895438

Bug 895438 - making status bar icons accessible to a screen reader. r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/6a8389dcaded
Author: Yura Zenevich <yzenevich@mozilla.com>
Desc: Bug 895438 - making status bar icons accessible to a screen reader. r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/93787fffdfa7
Author: Jose M. Cantera <jmcanterafonseca@gmail.com>
Desc: Merge pull request #19383 from jmcanterafonseca/ftu_final

Bug 1010782 - [FTU-Comms separation] FTU-Comms separation remaining task...

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https://hg.mozilla.org/integration/gaia-central/rev/b70b7faad49f
Author: Jose M. Cantera <jmcanterafonseca@gmail.com>
Desc: Bug 1010782 - [FTU-Comms separation] FTU-Comms separation remaining tasks
Bumping manifests a=b2g-bump