Make a field final.

Make a field that can be final, final.

Change-Id: I597ef43bad5045a63e7e9fe6eedd8e0f0a637ca4

core/java/android/net/LocalSocket.java

@@ -29,7 +29,7 @@ import java.net.SocketOptions;
  */
 public class LocalSocket implements Closeable {
 
-    private LocalSocketImpl impl;
+    private final LocalSocketImpl impl;
     private volatile boolean implCreated;
     private LocalSocketAddress localAddress;
     private boolean isBound;