Set WIFI icon for connection based on connection extra.

Previous methodology looked for changes in the Ims feature availability
in the ImsPhoneCallTracker.  I.e. there was an assumption made that if
the IMS stack had VoWIFI enabled that we were using it.

A new extra was added for the 3 nexus devices,
ImsCallProfile.EXTRA_CALL_RAT_TYPE.  This determines the radio technology
in use for a call, which we can then use to accurately show the WIFI
calling icon.

Bug: 20144385
Change-Id: I5aee9a571c7458f576a9208b81bed9781046a39a