Synchronize map access

Summary:
We want applying deltas to be an atomic operation, from incrementing the delta message ID to updating the relevant maps.
This is a simple approach to synchronize the corrsponding method.

We will probably need to go with a more sophisticated approach, that makes sure that deltas are applied in order. That would also allow us to lock only on writes.

Reviewed By: kathryngray

Differential Revision: D6846560

fbshipit-source-id: 175a80b4e39223883e397d75e20109fc12a2a878

ReactAndroid/src/main/java/com/facebook/react/devsupport/BundleDownloader.java

@@ -286,7 +286,7 @@ private static boolean storePlainJSInFile(BufferedSource body, File outputFile)
     return true;
   }
 
-  private boolean storeDeltaInFile(BufferedSource body, File outputFile) throws IOException {
+  private synchronized boolean storeDeltaInFile(BufferedSource body, File outputFile) throws IOException {
 
     JsonReader jsonReader = new JsonReader(new InputStreamReader(body.inputStream()));