Small telephony fixes before more radical changes

This commit contains small fixes to time zone detection docs, APIs and
types before a more radical refactoring that spans git projects and
branches.

Summary of changes:
1) Docs fixes.
2) Test cleanup.
3) Switch from java.util.TimeZone to android.icu.util.TimeZone type -
  the time zone detection code generally does this since it is a richer
  API and is generally "better".
4) Reduce the classes that "know" about libcore classes by add a method
  / using TimeZoneLookupHelper from MccTable.
5) Modify TimeZoneLookupHelper.OffsetResult: Use accessors rather than
direct field access, use TimeZone instead of String timeZoneId.

(5) is in anticipation of OffsetResult moving to a future System API.

Bug: 139091367
Test: atest com.android.internal.telephony.NitzStateMachineImplTest
Test: atest com.android.internal.telephony.TimeZoneLookupHelperTest
Change-Id: I7fea777d4cf0328ac26f46ba5b1802b91260b5b4