Simplify APIs exposed for time zone lookups

The libcore.timezone APIs may form the basis for some
new SystemApis. Before starting that process the API
surface is being rationalized to establish the core
use cases.

Test: Treehugger
Bug: 139091367
Change-Id: I6c8b791524d16223c414612913b22f4d70c27d61
Merged-In: I6c8b791524d16223c414612913b22f4d70c27d61
(cherry picked from commit 400efa36)