message: [tflite] make calling NNAPI work again
  calling PrepareOpsAndTensors() before using NN API looks
  1. unnecessary
  2. will decrease next_node_to_prepare_ so that the next
  next_node_to_prepare_ == nodes_and_registration_.size()
  will fail
diff: @@ -291,7 +291,6 @@ TfLiteStatus Interpreter::Invoke() {
  TfLiteStatus status = kTfLiteOk;
  if (nnapi_delegate_) {
  - TF_LITE_ENSURE_STATUS(PrepareOpsAndTensors());
  if (next_node_to_prepare_ == nodes_and_registration_.size()) {
  TF_LITE_ENSURE_OK(&context_, nnapi_delegate_->Invoke(this));
  return kTfLiteOk;
  