mv_machine_learning: replace char arrays by std::string

mv_machine_learning: replace char arrays by std::string

[Issue type] code refactoring

This patch is the first step in refactoring the
mv_machine_learning module to replace char* by std::string.
The refactoring will take place in a bottom-up fashion-
meaning that the individual task group classes will be refactored
first, and finally, the ITask class will be modified along with
all the adapter classes. This patch applies this change to the
ImageClassification task group.

Change-Id: If74f788758e2744bb9f1b27c6c607fbb41813512
Signed-off-by: Vibhav Aggarwal <v.aggarwal@samsung.com>