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

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https://hg.mozilla.org/integration/gaia-central/rev/878849f8053d
Author: Fernando Campo <fernando.campo@telefonica.com>
Desc: Merge pull request #19247 from fcampo/ussd-ui-send-button-992193

Bug 992193 - [Dialer] USSD notify/request specific UI management (r=etienne)

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https://hg.mozilla.org/integration/gaia-central/rev/301c37f1bd88
Author: Fernando Campo <fernando.campo@telefonica.com>
Desc: Bug 992193 - [Dialer] USSD notify/request specific UI management
- Hide button when no response expected
- Simple tests added
Bumping manifests a=b2g-bump