messageTray: Default to generic policy

How and if notifications are shown is controlled by NotificationPolicy objects. But ever since 098bd450, only notification daemon sources or notifications associated with an app are hooked up to GSettings.

The hardcoded default policy for built-in notifications (including those provided by extensions) arguably made sense back then, but now that the main setting has been rebranded as "Do Not Disturb" and is exposed prominently in the calendar drop-down, following GSettings is a better default.

Fixes #3291 (closed)