Start using NitzSignal.ageMillis

Start using NitzSignal.ageMillis in time detection. The main functional
change is one line in NitzStateMachineImpl.

The NITZ filtering has been updated, with some refactoring to try to
make things clearer:
1) The receipt time (ignoring age) is still used to tell if NITZ signals
are arriving too closely together. Arguments could be made both ways for
this, but it seems unimportant.
2) The UTC time check does take into the account the age, since it is
required to understand what UTC time the NITZ signal encodes.

Time zone detection logic is unaffected except for NITZ filtering.

Bug: 203638508
Test: atest tests/telephonytests/src/com/android/internal/telephony/nitz/
Test: atest tests/telephonytests/src/com/android/internal/telephony/NitzSignalTest.java
Test: treehugger
Change-Id: I868543a87bed8e8d7ff0a8b09e126f56cdbd2380