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
Change-Id: I489e5f2f8ff27da60fed215fa5f51326933fd79e