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: Ib615976fae76ceb98873a33e1e995542e691c9d0
Merged-In: Ib615976fae76ceb98873a33e1e995542e691c9d0
(cherry picked from commit babab5cd)