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

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https://hg.mozilla.org/integration/gaia-central/rev/dfa89b4c45fe
Author: Zac <zcampbell@mozilla.com>
Desc: Merge pull request #19021 from elefant/bug/996510-system-message-testcase

Bug 996510 - Add system message test case for Bug 906164

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https://hg.mozilla.org/integration/gaia-central/rev/51e271ee4f9a
Author: hchang <hchang@mozilla.com>
Desc: Bug 996510 - Add system message test case for Bug 906164
Bumping manifests a=b2g-bump