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

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https://hg.mozilla.org/integration/gaia-central/rev/28d4f48170b6
Author: punamdahiya <punamdahiya@yahoo.com>
Desc: Merge pull request #16093 from punamdahiya/Bug969475

Bug969475 - [Settings][Sound] - Fix of select ring tone and alert tone flow does not work in FF Nightly r=gduan

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https://hg.mozilla.org/integration/gaia-central/rev/57dbef8ed087
Author: Punam Dahiya <punamdahiya@yahoo.com>
Desc: Bug969475 - [Settings][Sound] - Fix of select ring tone and alert tone flow does not work in FF Nightly r=gduan

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https://hg.mozilla.org/integration/gaia-central/rev/8bfc90a66369
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #16037 from bebef1987/data_layer

Bug 968769 - Remove data_layer and accessibility from base.py's init

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https://hg.mozilla.org/integration/gaia-central/rev/eb1f4a5e5384
Author: Bebe <bebe@mozilla.ro>
Desc: Bug 968769 - Remove data_layer from base.py's init
Bumping manifests a=b2g-bump