[Settings] Support default value for APN protocol

Add default value on APN (roaming) protocol based on configuration when user leave it empty during editing APN.

Bug: 142440775
Test: manual
Test: make RunSettingsRoboTests -j ROBOTEST_FILTER=ApnEditorTest
Change-Id: I3f1ca0ef17ceaaf2a94b21b6034cd74e56081b61
2 files changed