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

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https://hg.mozilla.org/integration/gaia-central/rev/9e28669b2798
Author: Rudy Lu <left.lu@gmail.com>
Desc: Merge pull request #11044 from RudyLu/homescreen/Bug893966-add_1.5x_assets

Bug 893966 - some 1.5x assets are still missing for Homescreen app. r=rexboy7.

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https://hg.mozilla.org/integration/gaia-central/rev/2d7bfac436ec
Author: Rudy Lu <left.lu@gmail.com>
Desc: Bug 893966 - some 1.5x assets are still missing for Homescreen app
 1. Add 1.5x assets.
 2. Modify the background size for app delete icon.