add Do Not Ask Again button for wifi emergency notif.

A user reported that the emergency wifi calling notification was
spammy and had no option to never show it again. This change allows
users to permanently silence the notification.

Fixes: 290241725
Test: 1 new unit test + manual:
         (1) add sim to DUT
         (2) disable mobile data & roaming
         (3) set calling preference to WIFI calling
         (4) select a WIFI with bad connection
         observe: when the notif posts and the
           "Do Not Show Again" button is clicked,
            the notif is never shown again.

Change-Id: Icc853bc6a41cb83f68ace3f3fb8fc22b37c324f4