Reformat local numbers that are placing calls in Singapore to not show

the country code.

Due to local regulations and recommendations about only showing the +65
country code when being called by international numbers as a means to
combat international spam calls.

This change also restructures the check so that other countries can be
added if needed later on.

Flag: telephony.remove_country_code_from_local_singapore_calls

Bug: 284416645
Test: atest PhoneNumberUtilsTest, atest PhoneNumberUtilsTest#testFormatSingaporeInternational, atest PhoneNumberUtilsTest#testFormatSingaporeNational
Test2: manual validity checks on call/sms/data

Change-Id: I065709ecb77764e7c838f365f98dc70bbcf7d6cf