Add new secure setting for controlling Messaging notifications

Bug 5552615
We're adding a new intent to allow 3rd party apps to disable
the Messaging app's notification. We want any app to be able to query
the setting of whether sms notifications are on or off, but only allow
system apps, particularly the Messaging app, to change that setting.
Other apps can change the setting by firing off an intent which brings
up a dialog activity in the Messaging app that'll let them turn off
Messaging notifications.

Change-Id: I4d2721191b86010efb383a24c28d911496440657