Refactor NitzData.

Extract split pattern to static final field and use java.time.LocalDateTime
instead of java.util.Calendar.

Test: atest frameworks/opt/telephony/tests/telephonytests/src/com/android/internal/telephony/NitzDataTest.java

Change-Id: I69c7095f7ff3896b3042d96a737b8ecc2b62aa93