Made findCountriesByMcc use delegate like findNetworkByMccMnc

This code is used in the telephony codebase and it makes sense to have everything consistent rather than importing deps of i18n for half of the methods.

Bug: 376493245
Test: atest still passing
Flag: com.android.icu.telephony_lookup_mcc_extension

Change-Id: Ic526683b2587e367bb13fdfdb52d776861d2f57f