Add new system API constant DEFAULT_PREFERRED_NETWORK_MODE

Bug: 140768340
Test: Following tests have been done
atest packages/services/Telephony/tests/
atest frameworks/opt/telephony/tests/telephonytests/
Manual sanity to check basic telephony

Change-Id: Ia114e4546d88e294ed37b6632a1eae6a06fbf7bc