Bug 1051793 - [CMAS] Plug in real CMAS input to the CMAS application

- Add cellbroadcast system message handler
- Removed unused styling and testing UI
- Add localization properties
To be honest I don't know too, at least this one looks like the simplest one, one more option would be to store some additional info in _storage_ with the key that equals to ```notification.tag```, but probably here it's overkill