Tidy up libcore.timezone APIs

Tidy up the libcore.timezone APIs to make them as close as possible to
android.timezone. In future, these classes should be repackages to be
the actual android.timezone classes, so the APIs need to be in sync.

Bug: 148086409
Test: treehugger
Change-Id: Ia8e903655a843baeb539febfd201132be94cf7b1
10 files changed