[FlexBuffer][Java] ReadWriteBuf and ReadBuf interface public (#5948)

Those interfaces need to be public for use cases where the user pass a buffer to FlexBuffers class that is not a ByteBuffer.

This will fix #5944

java/com/google/flatbuffers/ReadWriteBuf.java

@@ -4,7 +4,7 @@ package com.google.flatbuffers;
  * Interface to represent a read-write buffer. This interface will be used to access and write
  * FlexBuffers message.
  */
-interface ReadWriteBuf extends ReadBuf {
+public interface ReadWriteBuf extends ReadBuf {
 
     /**
      * Clears (resets) the buffer so that it can be reused. Write position will be set to the