dbusService: Exempt org.gnome.Shell from sender tracking

If an interface has any signals, we don't want to auto-shutdown while a caller is still connected to a signal.

Unfortunately we can't tell whether there are any signal connections, so we track all callers instead, and keep the service alive while any of them is still on the bus.

For services that we call from gnome-shell itself - like screencasts or extensions - this has the unintended side effect of effectively disabling auto-shutdown.

Address this by exempting the org.gnome.Shell name from sender tracking.

Services that we expect to keep running for the lifetime of the shell already disable auto-shutdown, so the only downside is a small startup delay to resolve the well-known shell name.

Close #7250 (closed)