Ignore IWLAN RAT roaming mode in legacy mode

Certain Wifi Calling configurations for home and
roaming mode were causing devices to move between
IWLAN and cellular data registration repeatedly.
Instead, we should not change the roaming mode if
voice is not registered and data moves from LTE->
IWLAN.

Bug: 129114951
Test: atest FrameworksTelephonyTests:ImsPhoneTest
Change-Id: Ic3628e04e4cbcd6d51489a4c3081efed76de6758