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

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https://hg.mozilla.org/integration/gaia-central/rev/c873d0561b30
Author: EragonJ <eragonj@eragonj.me>
Desc: Merge pull request #17842 from EragonJ/bug-973440

Bug 973440 - [settings] refactor Screen lock panel with AMD pattern

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https://hg.mozilla.org/integration/gaia-central/rev/dc4c542a2875
Author: EragonJ <eragonj@eragonj.me>
Desc: Bug 973440 - [settings] refactor Screen lock panel with AMD pattern
Bumping manifests a=b2g-bump