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

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https://hg.mozilla.org/integration/gaia-central/rev/ab73c7cd16f4
Author: George <georgeiscoming@gmail.com>
Desc: Merge pull request #20264 from cctuan/938045-2

Bug 938045 - [Window Management] Implement AppSelection for Copy&Paste on Firefox OS

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https://hg.mozilla.org/integration/gaia-central/rev/8da6f4f3198c
Author: cctuan <georgeiscoming@gmail.com>
Desc: Bug 938045 - [Window Management] Implement AppSelection for Copy&Paste on Firefox OS
Bumping manifests a=b2g-bump