Make the NitzStateMachine behavior more regular

Much of the NitzStateMachine behavior was adopted from
an understanding of how ServiceStateTracker behaved, including behavior
that didn't make complete sense.

This commit improves guarantees about when fields are set to
provide clearer post-conditions from the various "handle" methods.

Specifically, after this commit:

mLatestNitzSignal - always holds the most recently received NITZ signal.
mSavedTimeZoneId - always holds the last time zone detected.
mNitzTimeZoneDetectionSuccessful - is always set when NITZ has been used
to set the time zone.

Before this commit the statements above were not always true, depending on
the order in which events were received.

Test: atest FrameworksTelephonyTests:com.android.internal.telephony.NitzStateMachineTest
Test: atest
FrameworksTelephonyTests:com.android.internal.telephony.ServiceStateTrackerTest
Bug: 78217059
Change-Id: Ib935431cf1a0871801635894451a68a3f1702272