When blocking direct access to the notification service, and relying on
libnotify 0.8's implicit portal support, we got those warnings:

(rhythmbox:2): libnotify-WARNING **: 15:02:43.424: Running in confined mode, using Portal notifications. Some features and hints won't be supported
libnotify-Message: 15:02:43.424: Category is not available when using Portal Notifications
libnotify-Message: 15:02:43.425: Category is not available when using Portal Notifications
libnotify-Message: 15:02:43.454: Category is not available when using Portal Notifications

We should only set the category when not using Flatpak.

See https://github.com/flathub/org.gnome.Rhythmbox3/pull/65