mv_machine_learning: introduce get_result_count API for image classification

mv_machine_learning: introduce get_result_count API for image classification

[Issue type] : new feature

Introduce get_result_count API for image classification task group.

In user perspective, this API provides information on how many results exist
so that user can request each result corresponding to a user-given index.
And also, in framework perspective, it provides consistent API behavior -
get_result_count API call updates _current_result of task group by calling
getOutput function of ITask, and get_result API call returns _current_result
value by calling getOutputCache function of ITask.

And we are enough with get_result_count and get_result API so drop existing
get_label API.

Change-Id: I9c1cb9e855494474af1510bbaf94febbdc57f05e
Signed-off-by: Inki Dae <inki.dae@samsung.com>