DO NOT MERGE 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 pick from commit 74264329da5c52fbbafc1d20457056fdeabe19dc)

Change-Id: I74702938694fea9b573524d1a2ae3c20ce52b54d
diff --git a/proto/src/metrics_constants.proto b/proto/src/metrics_constants.proto
index 81d645e..20cca8e 100644
--- a/proto/src/metrics_constants.proto
+++ b/proto/src/metrics_constants.proto
@@ -2219,6 +2219,26 @@
     // OS: N-MR2
     ACTION_LOCK_BECAUSE_SIM_REMOVED = 497;
 
+    // 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 N-MR2 Constants, all N-MR2 constants go above this line ----
 
     // Add new aosp constants above this line.