Java: Allow access to the underlying ByteBuffer from a table.

Change-Id: Id268e35de207c25c809a09071b81eea873c9123e
Tested: on Linux.

java/com/google/flatbuffers/Table.java

@@ -25,6 +25,8 @@ public class Table {
   protected int bb_pos;
   protected ByteBuffer bb;
 
+  public ByteBuffer getByteBuffer() { return bb; }
+
   // Look up a field in the vtable, return an offset into the object, or 0 if the field is not
   // present.
   protected int __offset(int vtable_offset) {