Adds SystemMessage for CarrierNetworkNotification

Adds a separate field in SystemMessage class for Carrier Network
Notification, to be able to differentiate between Carrier and Open
network notifications in case we have both notifications at the same
time.

Bug: 69310016
Test: Builds successfully
Change-Id: I5c4f40640a924e39cf4c6a33825afcc1a5c9d4f9