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