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

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https://hg.mozilla.org/integration/gaia-central/rev/a8529643a6f0
Author: EragonJ <eragonj@eragonj.me>
Desc: Merge pull request #15342 from EragonJ/bug-948847

Bug 948847 - [Gaia] add AirplaneModeHelper

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https://hg.mozilla.org/integration/gaia-central/rev/e0357cbb6302
Author: EragonJ <eragonj@eragonj.me>
Desc: Bug 948847 - [Gaia] add AirplaneModeHelper

- other apps can use AirplaneModeHelper to know current airplane status
- add a lot of related unit tests
- add two mozSettings key for airplane mode use
- update gaia-ui-tests