ImsPhone: defer intial roaming state update

The intial roaming update was in constructor of ImsPhone. It's so early
that settings provider and carrier config may not be loaded. Hence
incorrect WFC mode was got and set. Now we defer intial roaming update
to ImsManager#updateWfcFeatureAndProvisionedValues, invoked on
EVENT_CARRIER_CONFIG_CHANGED

Bug: 31972897
Change-Id: Id7a14f2d4ff0c638c53bcf7bbae2468750581e06