Reduce visibility of MultipartStreamReader

Summary:
In an attempt to reduce footprint of React Native Android public APIs we are reducing visibility of classes and interfaces that are not meant to be used publicly OR are public but have no usages.
As part of our analysis, which involved looking for usages inside the Meta codebase and code search in OSS, we've detected that this class/interface is public but it's not used from other packages.

If you are using this class or interface please comment in this PR and we will restate the public access.

changelog: [Android][Changed] Reducing visibility of MultipartStreamReader

bypass-github-export-checks

Reviewed By: arushikesarwani94

Differential Revision: D49803274

fbshipit-source-id: 7bcbf032b9ce59cd624e782ee81f69dad747bf8e

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/devsupport/MultipartStreamReader.java

@@ -15,7 +15,7 @@
 import okio.ByteString;
 
 /** Utility class to parse the body of a response of type multipart/mixed. */
-public class MultipartStreamReader {
+class MultipartStreamReader {
   // Standard line separator for HTTP.
   private static final String CRLF = "\r\n";