MLCE-103 Use ImageTensorGenerator in ModelAccuracyTool

MLCE-103 Use ImageTensorGenerator in ModelAccuracyTool

* Refactor ImageTensorGenerator into a header so that it can be used
inside ModelAccuracyTool. ModelAccuracyTool now can accept image files
instead of tensor text files as input. ImageTensorGenerator remains a
standalone tool for converting images into tensors text files.
* Inside the ImageTensorGenerator, use the existing image preprocessing
method InferenceTestImage::Resize which has the correct image normalization
techniques and other resize utilities.

Change-Id: Ia662fed4752fb81c5cfa6d15967c6aae4aaf1155
Signed-off-by: SiCong Li <sicong.li@arm.com>