Add a cache of the last CountryTimeZones looked up

There should be strong consistency of the country being
looked up so it makes sense to have a cache near to the point
of use. The TimeZoneFinder object currently caches the
last one retrieved, but that's general purpose and may
be invalidated more regularly (and it may be removed).

Test: atest FrameworksTelephonyTests
Bug: 63743683
Change-Id: If0b2aa851c78141bfdb85fd78129d70e94ed8c06