[Settings] Remove internal ImsManager access within MobileNetworkUtils

Remove the access of com.android.ims.ImsManager from MobileNetworkUtils.

Bug:  140542283
Test: m RunSettingsRoboTests -j
ROBOTEST_FILTER=WifiCallingSettingsForSubTest
Test: m RunSettingsRoboTests -j
ROBOTEST_FILTER=WifiCallingPreferenceControllerTest
Test: m RunSettingsRoboTests -j
ROBOTEST_FILTER=WifiCallingSettingsTest
Test: m RunSettingsRoboTests -j
ROBOTEST_FILTER=WifiCallingSliceHelperTest

Change-Id: Id2550a11c185ac44c1f70c98929c4b1d659f9a0c