code: TsiFrameProtector createFrameProtector(int maxFrameSize, ByteBufAllocator alloc) {
      unwrapper = null;
      return internalHandshaker.createFrameProtector(maxFrameSize, alloc);
    }
docs: Creates a frame protector from a completed handshake. No other methods may be called after the
  frame protector is created.
  
  @param maxFrameSize the requested max frame size, the callee is free to ignore.
  @return a new {@link io.grpc.alts.internal.TsiFrameProtector}.