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

(cherry picked from commit e1cbf15)

Change-Id: I6020c283329f752170cef451725b4e5287509bc5

proto/src/metrics_constants.proto

@@ -2218,6 +2218,30 @@ message MetricsEvent {
     // CATEGORY: QUICK_SETTINGS
     QS_NFC = 497;
 
+    // ---- End N-MR2 Constants, all N-MR1 constants go above this line ----
+
+    // ACTION: A captive portal was detected during network validation
+    // CATEGORY: NOTIFICATION
+    // OS: N-MR2
+    NOTIFICATION_NETWORK_SIGN_IN = 740;
+
+    // ACTION: An unvalidated network without Internet was selected by the user
+    // CATEGORY: NOTIFICATION
+    // OS: N-MR2
+    NOTIFICATION_NETWORK_NO_INTERNET = 741;
+
+    // ACTION: A validated network failed revalidation and lost Internet access
+    // CATEGORY: NOTIFICATION
+    // OS: N-MR2
+    NOTIFICATION_NETWORK_LOST_INTERNET = 742;
+
+    // ACTION: The system default network switched to a different network
+    // CATEGORY: NOTIFICATION
+    // OS: N-MR2
+    NOTIFICATION_NETWORK_SWITCH = 743;
+
+    // ---- End O Constants, all O constants go above this line ----
+
     // Add new aosp constants above this line.
     // END OF AOSP CONSTANTS
   }