messageTray: Default to generic policy

messageTray: Default to generic policy

How and if notifications are shown is controlled by NotificationPolicy
objects. But ever since 098bd45, 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.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3291

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1511>