Define Network notification proto constants.

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