This patch defines 4 new constants in metrics_constants.proto
corresponding to the 4 network notifications shown by
NetworkNotificationManager.
Estimates for the average number of notifications:
- SIGN_IN: 3/day/device
- NO_INTERNET: 5/day/device
- LOST_INTERNET: 5/day/device
- NETWORK_SWITCH: 3/day/device (turned off right now)
Test: no functional change
Bug: 32198726
Bug: 33030620
Change-Id: I31bd9b5562b94d62aac499f7174a3e115671bda8