Nudges: Move roaming nudge logic to wifi listener

When the call ends, if the user was on wifi the entire time and was roaming
then we will show the roaming text. Otherwise, this will count as a wifi
nudge.

Change-Id: Ic85a308d1860977fe4a62265fc293025ddb6a5a3
Ticket: CD-678
(cherry picked from commit 52cab04f)