Instrument captive portal login activity

Also add CAPTIVE_PORTAL_LOGIN_ACTIVITY_SSL_ERROR constants to
metrics_constants.proto for counting ssl error pages shown by the
captive portal login activity.

Bug: 36203355
Bug: 34901696
Bug: 36532213
Test: manually tested with captive portal testing setup while watching
      event logs: adb logcat -b events | grep captiveportallogin
        06-08 16:25:20.279 21558 21558 I sysui_action: [1004,com.android.captiveportallogin]
        06-08 17:01:04.052 21558 21558 I sysui_action: [1007,com.android.captiveportallogin]

Change-Id: Ia05853506d424ad4cb2c11770ae7718fbef3f01c
(cherry picked from commit c61dc92fc0619d101c4b789f6305012701cc47a0)
diff --git a/proto/src/metrics_constants.proto b/proto/src/metrics_constants.proto
index 11ea841..fa2b1ee 100644
--- a/proto/src/metrics_constants.proto
+++ b/proto/src/metrics_constants.proto
@@ -4090,6 +4090,11 @@
     // OS: O DR
     SETTINGS_ASSIST_GESTURE_FIRST_TIME = 1012;
 
+    // CaptivePortalLoginActivity displays SSL error page
+    // CATEGORY: GLOBAL_SYSTEM_UI
+    // OS: O DR
+    CAPTIVE_PORTAL_LOGIN_ACTIVITY_SSL_ERROR = 1013;
+
     // Add new aosp constants above this line.
     // END OF AOSP CONSTANTS
   }