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/ConnectToWifiHandler.java

@@ -17,6 +17,8 @@
 package com.android.settings.wifi.slice;
 
 import android.app.Activity;
+import android.net.ConnectivityManager;
+import android.net.Network;
 import android.net.wifi.WifiManager;
 import android.os.Bundle;
 
@@ -36,10 +38,15 @@ public class ConnectToWifiHandler extends Activity {
     public void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
 
+        final Network network = getIntent().getParcelableExtra(ConnectivityManager.EXTRA_NETWORK);
         final Bundle accessPointState = getIntent().getBundleExtra(
                 WifiDialogActivity.KEY_ACCESS_POINT_STATE);
 
-        if (accessPointState != null) {
+        if (network != null) {
+            final ConnectivityManager cm = getSystemService(ConnectivityManager.class);
+            // start captive portal app to sign in to network
+            cm.startCaptivePortalApp(network);
+        } else if (accessPointState != null) {
             connect(new AccessPoint(this, accessPointState));
         }
         finish();