API for retrieving time zone IDs by country

New method has been requested by Setup Wizard.

Test: CTS: run cts-dev -m CtsUtilTestCases -t android.util.cts.TimeUtilsTest
Bug: 116544863
Merged-In: I31a9e3d07d5c3fbc8ba1d9c9b398cb2661aa71f9
Change-Id: I31a9e3d07d5c3fbc8ba1d9c9b398cb2661aa71f9
(cherry picked from commit f9bb2d8f)