mv_machine_learning: code refactoring to checking custom device type

mv_machine_learning: code refactoring to checking custom device type

This patch does code refactoring for enhancing code readiability
of the condition which checks if config type path is required or
not according to given backend and target device types.

As for this, this patch adds IsValidBackendType fucntion to check if
a given backend type is valid or not, IsConfigFilePathRequired
function to check if config file path is required or not, and
makes these functions to be used instead of existing complicated condition.

Change-Id: I195e3caeb205b159e2128a14c2b4b6516e7b952d
Signed-off-by: Inki Dae <inki.dae@samsung.com>