Country tz lookup / expose CountyTimeZones

This commit moves CountryTimeZones to be a top-level class.
It also add a replacement for the telephony logic for looking
up a time zone given only an ISO code. The
isDefaultOkForCountryTimeZoneDetection() method logic has been
compared with the existing telephony tz lookup logic and it
returns the same answers for now and in 6 months.

Bug: 63743683
Test: vogar luni/src/test/java/libcore/libcore/util/CountryTimeZonesTest.java
Test: vogar luni/src/test/java/libcore/libcore/util/TimeZoneFinderTest.java
Change-Id: I817991657f02630db241a0e1134accddaf935b1c