[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/ReadBuf.java

@@ -3,7 +3,7 @@ package com.google.flatbuffers;
 /**
  *  Represent a chunk of data, where FlexBuffers will read from.
  */
-interface ReadBuf {
+public interface ReadBuf {
 
   /**
    * Read boolean from data. Booleans as stored as single byte