make label_image for tflite build again (#16206)

* make label_image for tflite build again

1. add namespace to label_image.h to make label_image for tflite build again
2. add --config monolithic and mention NDK settings in label_image.md
3. fix a typo in display_usage()

* relies on tensor type info to switch types

use tensor types of input and output tensors instead of command
line flag from user

* reformatted according to review