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

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https://hg.mozilla.org/integration/gaia-central/rev/f5f910291538
Author: GaryChen,PYChen <mmm198219@gmail.com>
Desc: Merge pull request #19418 from mpizza/Bug_952373_rename_HOMESCREEN_TO_SYSTEM

Bug 952373 - [build script] should rename HOMESCREEN to SYSTEMAPP in gaia build script. r=yuren.

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https://hg.mozilla.org/integration/gaia-central/rev/b48a3a07c20e
Author: mpizza <mmm198219@gmail.com>
Desc: Bug 952373 - [build script] should rename HOMESCREEN to SYSTEMAPP in gaia build script
Bumping manifests a=b2g-bump