[Settings] Hidden API remove - WFC mode settings

Change API from getWfcMode/setWfcMode into APIs within ImsMmTelManager.

Bug: 140542283
Test: make RunSettingsRoboTests -j ROBOTEST_FILTER=WifiCallingPreferenceControllerTest
make RunSettingsRoboTests -j ROBOTEST_FILTER=WifiCallingSettingsForSubTest
make RunSettingsRoboTests -j ROBOTEST_FILTER=WifiCallingSliceHelperTest
atest WifiCallingSettingUiTest

Change-Id: I2ee01852cb4ee90fe9db256d3333f461cc070da5