Make use of the default time zone for a country

This change adds code to expose the country default
time zone ID added in previous changes. This change
makes the attribute mandatory.

There have been some structural changes in
TimeZoneFinder to delay creation of TimeZone objects
when not needed and to enable lookups with ISO codes
that are not lower-case. There's a lot of code that
currently assumes lower-case lookups but making it
work when the codes are upper case (as used in
Locale and by zone.tab) should do no harm.

Tests have been added to TimeZoneFinderTest for the
new behavior.

Additional tests have been added to ICUTest to confirm
that ICU offers a superset of time zone IDs over the
Android ZoneInfoDB IDs (which backs java.util.TimeZone).
Note: Bug 30277331 means Asia/Hanoi is an issue for ICU
but that ID is a known issue and unlikely to be used
in practice.

Test: Booted device
Test: CTS: run cts -m CtsLibcoreTestCases -t libcore.icu.ICUTest
Test: CTS: run cts -m CtsLibcoreTestCases -t libcore.util.TimeZoneFinderTest
Bug: 30277331
Bug: 65575839
Bug: 34266809
Change-Id: I5210df5fa8e25acdfc819a42b03feccbce60750d