datetime: check CanNTP when initializing the panel

If the system does not support NTP (as determined by the presence of an
NTP unit in /usr/lib/systemd/ntp-units.d), the Automatic Date & Time row
is intended to be hidden. We respond to changes to timedated's CanNTP
property, but we never account for the initial state of the property,
so we wind up displaying the Automatic Date & Time row even though the
switch is guaranteed to be broken.

Check CanNTP when initializing the panel to fix this.

https://bugzilla.gnome.org/show_bug.cgi?id=732021