Add prompt to sign in at captive portal to Wi-Fi Slice

- Support signing in captive portal APs in WifiSlice
- Show ContextualWifiSlice when signing in is required
- Generifies SliceBackgroundWorker.getInstance() to return <T extends SliceBackgroundWorker>

Fixes: 128056349
Test: make RunSettingsRoboTests -j
Change-Id: Ib4d3942591a65e81018389e4c0bbddfea6854dbc
(cherry picked from commit dd9f92280b)

src/com/android/settings/wifi/slice/ContextualWifiSlice.java

@@ -57,7 +57,7 @@ public class ContextualWifiSlice extends WifiSlice {
             sActiveUiSession = currentUiSession;
             sPreviouslyDisplayed = false;
         }
-        if (!sPreviouslyDisplayed && !TextUtils.equals(getActiveSSID(), WifiSsid.NONE)) {
+        if (!sPreviouslyDisplayed && hasWorkingNetwork()) {
             Log.d(TAG, "Wifi is connected, no point showing any suggestion.");
             return null;
         }
@@ -67,4 +67,8 @@ public class ContextualWifiSlice extends WifiSlice {
 
         return super.getSlice();
     }
+
+    private boolean hasWorkingNetwork() {
+        return !TextUtils.equals(getActiveSSID(), WifiSsid.NONE) && !isCaptivePortal();
+    }
 }