Synchronize on FileSubscription.this

Regression from aws#519

Author: dagnir

core/src/main/java/software/amazon/awssdk/core/async/FileAsyncRequestBody.java

@@ -234,7 +234,7 @@ public void completed(Integer result, ByteBuffer attachment) {
                         closeFile();
                     }
 
-                    synchronized (this) {
+                    synchronized (FileSubscription.this) {
                         writeInProgress = false;
                     }
                 }