Bring back JS bundle loading progress bar on Android

Summary: The progress bar on Android was disabled in D5329011 because of T19653381, but was never enabled again. I spent some time trying to reproduce the issue of the bundle being stuck while loading, but didn't succeed. So let's enable the progress bar and monitor whether people would start seeing this bug again.

Reviewed By: cpojer

Differential Revision: D17148134

fbshipit-source-id: 5130b809460bc743d26a6e88961f81061089fe1d

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

@@ -109,11 +109,7 @@ public void downloadBundleFromURL(
     final Request request =
         requestBuilder
             .url(formatBundleUrl(bundleURL))
-            // FIXME: there is a bug that makes MultipartStreamReader to never find the end of the
-            // multipart message. This temporarily disables the multipart mode to work around it,
-            // but
-            // it means there is no progress bar displayed in the React Native overlay anymore.
-            // .addHeader("Accept", "multipart/mixed")
+            .addHeader("Accept", "multipart/mixed")
             .build();
     mDownloadBundleFromURLCall = Assertions.assertNotNull(mClient.newCall(request));
     mDownloadBundleFromURLCall.enqueue(