Replace getNetworkPrefixLength() with public API

Use public API getPrefixLength() instead

Test: build pass

Change-Id: Ib85aa12dc22ac5b03c9bb7101beaf340014eb4fd
diff --git a/src/com/android/settings/wifi/WifiConfigController.java b/src/com/android/settings/wifi/WifiConfigController.java
index 0be7c2b..793ba4c 100644
--- a/src/com/android/settings/wifi/WifiConfigController.java
+++ b/src/com/android/settings/wifi/WifiConfigController.java
@@ -1283,7 +1283,7 @@
                         mIpAddressView.setText(
                                 staticConfig.ipAddress.getAddress().getHostAddress());
                         mNetworkPrefixLengthView.setText(Integer.toString(staticConfig.ipAddress
-                                .getNetworkPrefixLength()));
+                                .getPrefixLength()));
                     }
 
                     if (staticConfig.gateway != null) {