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)

Change-Id: I9184f6263a003c2be28f41a550c6164c8d6ee907