sharing: Fix Remote Desktop on switch handling

Previously, we were only checking if the gnome-remote-desktop user service was running but that service is always running as of gnome-remote-desktop@7f4ef31f

Closes: #1775 (closed)

This looks like a sufficiently minimal patch to fix this issue. There was a suggestion that we could also test to maek sure the tls-key and tls-cert gsettings keys look normal enough. As long as people only enable RDP through gnome-control-center, then the gsettings value should be correct.