[Java][FlexBuffers] Make FlexBuffersBuilder reusable by adding clear() (#5889) (#5890)

java/com/google/flatbuffers/ByteBufferReadWriteBuf.java

@@ -12,6 +12,11 @@ public class ByteBufferReadWriteBuf implements ReadWriteBuf {
     this.buffer.order(ByteOrder.LITTLE_ENDIAN);
   }
 
+  @Override
+  public void clear() {
+    buffer.clear();
+  }
+
   @Override
   public boolean getBoolean(int index) {
     return get(index) != 0;