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

This reverts commit d8596e523c2d88b6190f242866c52864f76cdfe9, reversing
changes made to 409833a50b449339599d4071a637ef8f099fe2ab.
Bug 1051788 - 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