Previously the only thing preventing default-unmanaged devices from
being auto-activated was luck and the fact that they didn't have any
available connections when in the UNMANAGED state.  That's no longer
true, so we must be more explicit about their behavior.

Furthermore it makes no sense to allow default-unmanaged devices
to set priv->autoconnect=TRUE since that is never supposed to
happen, so enforce that both in NM itself and if the change
request comes in over the D-Bus interface.

Lastly, internal priv->autoconnect=TRUE changes never emitted a
property change notification, meaning the NMPolicy would never
schedule an autoconnect check if the device's priv->autoconnect
was set to TRUE as a result of re-activating or waking from sleep.