Use upper case country codes in Country.java

Previously external/libphonenumber accepted lower case
country codes (e.g. "us") but now it doesn't for performance reason.
Actually ISO 3166-1 doesn't allow lower cases, so we should not rely
on them.

Need to fix unit tests for PhoneNumberUtils, as it implicitly
relies on the previous behavior.

See also I3a3e6db84ed0d24290b1be19651fa9a82de4cc39

Bug: 4941319
Change-Id: If16f6531f274a0faf5e28724854409ca9b00a669