Bug 877751 - Handle Cell Broadcast System messages in the system app.

We do not distinguish between message classes as specified (and
previously implemented).

If a message is received while in the lockscreen then a notification
is sent which wakes the screen, sounds a notification and displays
it on the lock screen, clicking on the notification will then show
a dialog with a full message.

If we are currently using the device the dialog will be displayed
immediately.