Reduce duplication of time zone detection logic

Over the a series of prior commits the time zone detection behavior
when receiving ...

(a) an NITZ signal then a country code
and
(b) a country code and then an NITZ signal

... has been nudged closer together. The two paths are now
effectively identical.

Now it is possible to deduplicate the two paths. This change
merges parts of handleNetworkCountryCodeSet() with
handleTimeZoneFromNitz() into a new private
updateTimeZoneFromCountryAndNitz() method that shares common
NITZ + country code time zone detection logic.

The logging has generally been tidied as part of this change.

More tests have been added to cover the cases when the device
is on a test network.

Bug: 78217059
Test: atest  FrameworksTelephonyTests:com.android.internal.telephony.NitzStateMachineTest
Change-Id: If485f441dd766f2140ce248b793b3a8770f00404