Bug 947142 - [DSDS][Message] Message Report show SIM information
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.
Follow-ups:

* remove empty suite in settings_test.js
* move a stub creation in a setup in settings_test.js
* use the '_' alias for mozL10n.get in all the sms-received message handler
* remove the wallpaper request for the 'pick' activity
Fixing per comments