Remove NitzStateMachine.getSavedTimeZoneId()

Remove the NitzStateMachine.getSavedTimeZoneId() method.

The reasoning is as follows:
1) It is only used from one place.
2) The logic that uses it appears unnecessarily complicated.
3) Preserving the "get me the last "high quality" time zone worked out
by NitzStateMachine will be difficult to do once the judgement around
quality moves to the system server.
4) It's not clear how critical this one code path is.

Before this change, ServiceStateTracker preferred using
getSaveTimeZoneId() (which returns the ID of the last "resolved" time
zone) over using information from the last NITZ data received via
getCachedNitzData(). However, the time zone ID is determined by using
the NITZ data. The later call to:

mHbpcdUtils.getMcc(sid, utcOffsetHours, (isDst ? 1 : 0), isNitzTimeZone)

is basically using information available from the NITZ signal but arrived
at indirectly.

It doesn't seem too bad to just use the original information provided by
the carrier. The code has been simplified to only use
getCachedNitzData() and isNitzTimeZone boolean is now always set to true
when getMcc() is called and NITZ data was used.

Bug: 140712361
Test: treehugger
Change-Id: I218c4e441baa78b5ad9876e566b128202f49be35