Call DPM isPreferentialNetworkServiceEnabled

* API method isPreferentialNetworkServiceEnabled
  exists in DevicePolicyManager instead of
  UserManager

Bug: 186451999
Test: manual testing with TestDPC
Change-Id: Ib677f43c6552317d39c25b888b7a68adc7cd7270

Author: Alex Johnston

app/src/main/java/com/afwsamples/testdpc/DevicePolicyManagerGatewayImpl.java

@@ -519,7 +519,7 @@ public boolean isPreferentialNetworkServiceEnabled() {
         String method = "isPreferentialNetworkServiceEnabled";
         try {
             // TODO(b/179160578): use proper method when available on SDK
-            return (Boolean) ReflectionUtil.invoke(mUserManager, method);
+            return (Boolean) ReflectionUtil.invoke(mDevicePolicyManager, method);
         } catch (ReflectionIsTemporaryException e) {
             Log.wtf(TAG, "Error calling " + method + "()", e);
             return false;