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

src/com/android/settings/wifi/ConnectedAccessPointPreference.java

@@ -25,7 +25,6 @@ import androidx.preference.PreferenceViewHolder;
 
 import com.android.settings.R;
 import com.android.settingslib.wifi.AccessPoint;
-import com.android.settingslib.wifi.AccessPointPreference;
 
 /**
  * An AP preference for the currently connected AP