Reduce NTP polling interval

Change NTP polling interval from 24 hours to 18 hours. The max allowed
age of a time suggestion for it to be considered by the time_detector
is set to 24 hours, so if we only refresh NTP after 24 hours, it means
devices may commonly switch to newer NITZ signals when the NTP signal
"expires". Then it will flip back to NTP when the NTP alarm triggers.
Whether it switches to an NITZ signal or not also depends on things
like whether NITZ signals are received, or other things happen that
force the time_detector to reassess suggestions, after the NTP
suggestion has expired.

If we can keep devices consistently using NTP, that's going to lead to
 better results, especially given NITZ can be quite inaccurate.

18 hours has been chosen as an arbitrary compromise: it gives the
device 25% of the expiry interval to get a fresh signal. It should
mean that NITZ isn't used in the common case.

Bug: 217357882
Test: None
Change-Id: I86281888ce009a19e50240d7f07aac1f5ce72222