Annotate time zone classes to be usable by module libraries

The telephony stack is becoming a mainline module, and needs to be able to use these classes. This change adds annotations to mark that they are SystemAPI for modules only.

Flag: android.timezone.flags.expose_time_zone_system_api
Test: Build system.img
Bug: 432239933
Bug: 432471550
Change-Id: Ia8331abc3bc20e4be767167fde381073a1baab68