Add Nat64Prefix to LinkProperties copy constructor

The attribute is copied when parceling/unparceling so should also be
preserved in the copy constructor.

Test: tests based on this pass
Change-Id: Ia48d94fc075e6414f537dad181a236be5a32b41b

core/java/android/net/LinkProperties.java

@@ -191,6 +191,7 @@ public final class LinkProperties implements Parcelable {
             }
             setMtu(source.mMtu);
             mTcpBufferSizes = source.mTcpBufferSizes;
+            mNat64Prefix = source.mNat64Prefix;
         }
     }