Whitelist free sms shortcode 76551 in Mexico for constlletion.

Constellation team has acquire a free shortcode 76551 in Mexico
for contellation MO verification. We don't want to scare people
by poping up the non-necessary warning dialog.
BUG: 65050702
Test: manual

Change-Id: Iab79032f31f99c80a750e02be0f1e6c32aa85363