activation: Allow changing controller of exposed active connection
When activating a port with its controller deactivating by new
activation, NM will register state-change
signal waiting controller to
have new active connections. Once controller got new active connection,
the port will invoke nm_active_connection_set_controller()
which lead
to assert error on
g_return_if_fail(!nm_dbus_object_is_exported(NM_DBUS_OBJECT(self)))
because this active connection is already exposed as DBUS object.
To fix the problem, we remove the restriction on controller been write-only and notify DBUS object changes for controller property.