Makes captive portal server calculation in one place.

This also creates a hidden api for the captive portal server calculation
so that the Setup Wizard can use this as well.

bug:13246857
Change-Id: I4dfd0916df97cfce13252c7cc15f7bd05ed95f77
diff --git a/services/core/java/com/android/server/ConnectivityService.java b/services/core/java/com/android/server/ConnectivityService.java
index 65a27c8..37a6c02 100644
--- a/services/core/java/com/android/server/ConnectivityService.java
+++ b/services/core/java/com/android/server/ConnectivityService.java
@@ -4848,6 +4848,11 @@
     }
 
     @Override
+    public String getCaptivePortalServerUrl() {
+        return NetworkMonitor.getCaptivePortalServerUrl(mContext);
+    }
+
+    @Override
     public void startNattKeepalive(Network network, int intervalSeconds, Messenger messenger,
             IBinder binder, String srcAddr, int srcPort, String dstAddr) {
         enforceKeepalivePermission();