Merge pull request #17087 from etiennesegonzac/bug-980984-incoming-alert-from-system

Bug 980984 - Alert the user of an incoming from the system app to improve performance r=alive, rik
(cherry picked from commit 5aa7d6e424c46c2a9761ec85a016f39de3105d71)

Conflicts:
	apps/communications/dialer/test/unit/calls_handler_test.js
	apps/system/index.html
	apps/system/js/bootstrap.js
	apps/system/manifest.webapp
	apps/system/test/unit/bootstrap_test.js