[infer][java] add model for java.io.StringWriter which does not need to be closed

Summary:
The class StringWriter implements Closeable but does not need to be closed.

infer/models/java/src/java/io/StringWriter.java

@@ -0,0 +1,19 @@
+/*
+* Copyright (c) 2015 - present Facebook, Inc.
+* All rights reserved.
+*
+* This source code is licensed under the BSD style license found in the
+* LICENSE file in the root directory of this source tree. An additional grant
+* of patent rights can be found in the PATENTS file in the same directory.
+*/
+
+package java.io;
+
+public class StringWriter implements Closeable{
+
+  public StringWriter() {}
+  public StringWriter(int initialSize) {}
+
+  public void close() {}
+
+}