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

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https://hg.mozilla.org/integration/gaia-central/rev/991987e7a968
Author: Etienne Segonzac <etienne@segonzac.info>
Desc: Merge pull request #12658 from Rik/polish-statusbar-921056

Bug 921056 - Visual polish for status bar r=etienne

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https://hg.mozilla.org/integration/gaia-central/rev/31b1540e739e
Author: Anthony Ricaud <anthony@ricaud.me>
Desc: Bug 921056 - Visual polish for status bar

- CallScreen.isStatusBarMode to be DRY-er
- HandledCall.formatPhoneNumber will format according to isStatusBarMode
- Don't display lockscreen elements and group call details in status bar
- Use new icons
- Adjust spacing
- Held mode should be dark gray