Centralize knowledge of time zone file paths

Centralize the knowledge of time zone file paths. The
knowledge is in a couple of places and there is some
file name duplication. A new file will be
added if time_zones_by_country.xml is made
updateable outside of an OTA.

Test: make / boot (to confirm the new System class dependency is ok)
Test: run cts -m CtsLibcoreTestCases -t libcore.java.lang.SystemTest
Test: run cts -m CtsLibcoreTestCases -t libcore.util.TimeZoneDataFilesTest
Test: run cts -m CtsLibcoreTestCases -t libcore.util.ZoneInfoDBTest
Bug: 25338903
Change-Id: Iaf9afc148faee8db7839513270a621a8149fd5fa