Update WriteWifiConfigToNfcDialog.

- Use the new WifiManager API to retrieve the WPS NFC token for the
current network.
- Add WifiManagerWrapper class to support testing because Robolectric
does not yet support testing Android O.

Bug: 35725168
Test: m RunSettingsRoboTests
Change-Id: I7805bdcbe02dc262083bcd371d4ad88d256f8089

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

@@ -0,0 +1,23 @@
+package com.android.settings.wifi;
+
+import android.net.wifi.WifiManager;
+
+/**
+ * Wrapper around {@link WifiManager} to facilitate unit testing.
+ *
+ * TODO: delete this class once robolectric supports Android O
+ */
+public class WifiManagerWrapper {
+    private final WifiManager mWifiManager;
+
+    public WifiManagerWrapper(WifiManager wifiManager) {
+        mWifiManager = wifiManager;
+    }
+
+    /**
+     * {@link WifiManager#getCurrentNetworkWpsNfcConfigurationToken}
+     */
+    public String getCurrentNetworkWpsNfcConfigurationToken() {
+        return mWifiManager.getCurrentNetworkWpsNfcConfigurationToken();
+    }
+}