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

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https://hg.mozilla.org/integration/gaia-central/rev/4919bf19bc3f
Author: Eitan Isaacson <eitan@monotonous.org>
Desc: Merge pull request #18459 from eeejay/bug-996284

Bug 996284 - Give lockscreen icons a higher z-index than slider. r=snowmantv

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https://hg.mozilla.org/integration/gaia-central/rev/5454021d9320
Author: Eitan Isaacson <eitan@monotonous.org>
Desc: Bug 996284 - Give lockscreen icons a higher z-index than slider.
Bumping manifests a=b2g-bump