Add SystemMessages protos to Connectivity

This follows the model used for the same protos by the Wifi module in
service-wifi.

SystemMessageProto is used by NetworkNotificationManager to define IDs
for notifications that are also counted in system notification metrics.

Bug: 171860710
Test: m
Change-Id: Iec674913fad9ad59c04249714a08b5a0d8ab0f84