applet: fix status icon when a VPN has the default route

After commit 35736dd2 ("applet: use default active interface, even
if it is of unsupported class") when a VPN with default route is
active, applet_get_default_active_connection() returns the VPN
connection, and no icon would be set because the tun device doesn't
have a NMADeviceClass.

Require a device class only when we need to determine the status icon,
otherwise accept any connection.

Fixes: 35736dd2

https://bugzilla.gnome.org/show_bug.cgi?id=785775
https://bugzilla.redhat.com/show_bug.cgi?id=1471510