Bumping gaia.json for 2 gaia-central revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/28f428a7e830
Author: GaryChen(pychen) <mmm198219@gmail.com>
Desc: Merge pull request #13558 from mpizza/Bug_936418_localized

Bug 936418 - [keyboard] 'Layout selection' in keyboard style switcher di... r=rudyl.

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https://hg.mozilla.org/integration/gaia-central/rev/6cfa4a799833
Author: mpizza <mmm198219@gmail.com>
Desc: Bug 936418 - [keyboard] 'Layout selection' in keyboard style switcher dialog isn't localized