Tidy-up changes to time zone classes

Tidy up changes before removing /system tzdata file dependency.
Most of these changes are to improve consistency and duplication.
No functional changes are intended.

Bug: 131239046
Test: atest CtsLibcoreTestCases:libcore.libcore.timezone.ZoneInfoDBTest
Test: atest CtsLibcoreTestCases:libcore.libcore.icu.TimeZoneIntegrationTest
Change-Id: Idc86099fc6e18b46e04e63baaa4dbeec1a766d6b
6 files changed