[Settings] Support default APN (roam) protocol

1. Add "apn.settings_default_apn_protocol_string" config which
controls default value of APN protocol field when updating or creating an APN.
2. Add "apn.settings_default_apn_roaming_protocol_string" config which
controls default value of APN roaming protocol field when updating or creating an APN.
3. Add PROTOCOL_IPV4, PROTOCOL_IPV6 and PROTOCOL_IPV4V6 for options
available

Bug: 142440775
Test: manual
Change-Id: I2b13d281adbade4ed0b373d62cadd9252a147abb