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

* 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
Merge pull request #22732 from julienw/1051788-system-cmas

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