tflite_benchmark_model supports `use_nnapi` parameter. (#2337)

tflite_benchmark_model supports `use_nnapi` parameter. (#2337)

Previously tflite_benchmark_model always run models with interpreter only.
Now, it uses nnapi delegate on `use_nnapi=true`

```
LD_LIBRARY_PATH=Product/out/lib Product/out/bin/tflite_benchmark_model \
--graph=../models/mobilenet_quant_v1_224.tflite \
--input_layer="Placeholder" --input_layer_shape="1,224,224,3" \
--num_threads=1 --use_nnapi=true \
```

Note that it is still under development.
Instrument code for each operation is not inserted yet.

Signed-off-by: Sanggyu Lee <sg5.lee@samsung.com>