Set model path according to MLAPI backend

Set model path according to MLAPI backend

NNFW - in-house NN Runtime - needs NNPackage type of package
which is a directory containing a model file and its meta file.
For more details, you can refer to
https://github.com/Samsung/ONE/tree/master/nnpackage/examples/one_op_in_tflite

ML Single API framework of NNStreamer receives a full path of a given model file
from user - in our case, inference-engine-mlapi backend - and find metadata
in the directory that the a given model file is located.

So inference-engine-mlapi backend should pass a full path of
the a given model file to ML Single API framework.

Change-Id: I6bdd871d5b683dbd6e60fce0f6dbd052985cd514
Signed-off-by: Inki Dae <inki.dae@samsung.com>