Fix screencast support

Quoting the main commit:

dbusService/screencast: Hold during gstreamer checks

Some gstreamer plugins require a connection to the display server,
so they block until the server is up and running. That's why we
moved the check into the D-Bus service, so that the blocking would
not lock up the compositor itself.

However the block can still delay the service initialization so
much that auto-shutdown hits immediately when returning from the
constructor. If that happens, the proxy on the shell side is no
longer backed by a remote object when the init callback runs, and
all properties therefore resolve as `null`.

As a result, gnome-shell thinks that screencasts aren't supported
and hides the screencast button.

Fix this by holding the service during the gstreamer checks, so
that the auto-shutdown timeout only starts after the service is

Fixes #6051 (closed)