datetime: Improve NTP switch handling fixing update loop

We cannot simply set the state/active property of the switch without also triggering the notify:: or state-set signals at the same time. As such, we need to block the "set-state" handler when we update the "state" of the switch.

Also, while at it, change the switch to react to "state-set" and only update the "state" after we received the response from the system.

Fixes: #1299 (closed)