timesync: fix PropertiesChanges signals for NTP properties

timesync: fix PropertiesChanges signals for NTP properties

As in their current form they didn't work at all:

systemd-timesyncd[190115]: Assertion 's' failed at src/libsystemd/sd-event/sd-event.c:3058, function sd_event_source_set_enabled(). Ignoring.
systemd-timesyncd[190115]: Failed to reenable system ntp server change event source!
systemd-timesyncd[190115]: Failed to enable ntp server defer event, ignoring: Invalid argument

This was also pointed out in the post-merge review [0].

Let's address this together with the rest of the comments, and add
some tests to make sure everything works as it should.

Resolves: #28770
Follow-up to: 8f1c446

[0] https://github.com/systemd/systemd/commit/8f1c4469793f2f0281fdfbc20ba4085e20cdd16f#r124147466