DO NOT MERGE 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]

Merged-In: Ia05853506d424ad4cb2c11770ae7718fbef3f01c

(cherry picked from commit c61dc92f)
(cherry picked with DO NOT MERGE because oc-dev-plus-aosp already
contains a version of this commit compatible with proto2).

Change-Id: I55ea3c61842f16bf6a98e812fb6dc940f96f895c