Bug 1026685 - Silent and priority notifications for CMAS alerts r=mhenre...

...tty

* add a list of applications whose notifications should be silent
* add a list of applications whose notifications have more priority and stay on
  top of the notification screen
* Also reorganized some unit tests
actually I'd rather use the full hardcoded string, it's easy enough to find-and-replace if we change it at one point. But I followed @alivedise advice here ;)