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

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https://hg.mozilla.org/integration/gaia-central/rev/7828beb0a08a
Author: Carsten Book <tomcat@mozilla.com>
Desc: Merge pull request #24103 from oklang/Add-themeable-permission-to-gaia-apps

Bug 1067992 - Add themeable permission to relevant gaia apps. r=fabrice

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https://hg.mozilla.org/integration/gaia-central/rev/cc4322b7524e
Author: Olle Klang <olle.klang@sonymobile.com>
Desc: Bug 1067992 - Add themeable permission to relevant gaia apps.

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https://hg.mozilla.org/integration/gaia-central/rev/19e89b0a977b
Author: gasolin <gasolin@gmail.com>
Desc: Merge pull request #24156 from gasolin/issue-817659

Bug 817659 - [Settings] Settings - Sound -Slider default value not at 50..., r=eragon

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https://hg.mozilla.org/integration/gaia-central/rev/eac78798eee1
Author: gasolin <gasolin@gmail.com>
Desc: Bug 817659 - [Settings] Settings - Sound -Slider default value not at 50%
Bumping manifests a=b2g-bump