Add subtext for Wi-Fi items on Wi-Fi slice

- Sync the Wi-Fi slice items subtexts with Wi-Fi page, and add
  "Not connected" when the returned subtext is empty
- Wi-Fi slice header subtext just simply reveals on/off state
- Add a placeholder to the loading row to sync its height with other
  rows
- Show contextual Wi-Fi slice when the connected network is captive
  portal, is invalidated, or has limited connectivity

Bug: 130751985
Test: make RunSettingsRoboTests -j ROBOTEST_FILTER=com.android.settings.wifi
Change-Id: I0b2715295f00edc8dd7d6d75b7fdc18263675797