Bumping gaia.json for 1 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/19db38e5c333
Author: Julien Wajsberg <felash@gmail.com>
Desc: Bug 947180 - [DSDS][Message] Message notification informs of SIM r=schung

This shows a prefix in the message notification when we are in a Dual SIM
scenario.

This also introduces an utility function to find the correct SIM index and to
detect the Dual SIM scenario.
Bumping manifests a=b2g-bump