Fix NPE in CallerInfo.getCurrentCountryIso().

Internal method CallerInfo.getCurrentCountryIso() can throw NPE
if the country detector's detectCountry() method returns null.
Add code to check that the Country is not null before calling
getCountryIso() on it. Change fallback logic to call
locale.getCountry() whenever countryIso was not assigned.

Bug: 10607906
Change-Id: I08fe3f4d942f67c37a2e6ff0b067fe32ad8a6fa5