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

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https://hg.mozilla.org/integration/gaia-central/rev/9271d94f35a5
Author: Anthony Ricaud <anthony@ricaud.me>
Desc: Merge pull request #14577 from Rik/stop-ringtone-power-button-890434

Bug 890434 - Impossible to silence an incoming call without rejecting it r=etienne

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https://hg.mozilla.org/integration/gaia-central/rev/bae67c5e1267
Author: Anthony Ricaud <anthony@ricaud.me>
Desc: Bug 890434 - Impossible to silence an incoming call without rejecting it

We use Inter App Communication to make this work
- System app sends a message in 'dialercomms' when we press the power or volumedown buttons
- The Call Screen stops ringing and vibrating when receiving this message

I moved the Audio mock to shared because it is now used by 3 apps.
Had to modify some sms tests to work with the new shared mock.