[Bug 912508] Implement "X has left the call" banner.

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First approach, style needed
Change approach, working with event
Style fixes
Cleaning
Moved banner to call screen
Added tests
Revert "Revert "Merge pull request #12000 from nullaus/operator-variant""

This reverts commit 8b15a3359fc30f3703808763e976cfcab3812724.
Bug 898409, 898410: Fixes to operator variant

Fix writable property of navigator.mozPower, remove unused stubPower variable
Change operator variant helper from mobileConnection to iccHelper
mobileConnection seems to be unstable when the sim card is locked. This patch changes operator variant to use IccHelper for fetching mcc/mnc instead.
Remove listener on application of settings
Bug 891725 - Added unit tests for cell broadcast and voicemail customization