New Metrics constants for captive portal related UI

This patch adds 5 new constants in metrics_constants for counting
various UI interactions involving captive portals:
  - sign-in button in wifi network details card
  - captive portal login activity start up
  - captive portal login activity menu interactions (2 constants)
  - captive portal login activity auto-closes on portal dismissed

Bug: 34901696
Bug: 26303355
Test: compiled, flashed
Merged-In: I489e5f2f8ff27da60fed215fa5f51326933fd79e

(cherry pick from commit 11da42bb)

Change-Id: Ifc553b08a65e78a630e235de087b7659a228bec8