mv_machine_learning: face recognition: consider multiple threads

mv_machine_learning: face recognition: consider multiple threads

[Issue type] : new feature

Considered multiple threads to face recognigion task API.

In application, it can request an inference and register even unregister
by each thread context. So make sure to guarantee such requests using mutex.

And also this patch introduces test_face_recognition_multi_threads app
which adds two test cases, InferenceAndRegisterShouldBeOk
and InferenceAndRegisterAndUnregisterShouldBeOk, to test multiple threads.

Change-Id: I9217a572f6746591eeda774f113e9f2b23c82a8a
Signed-off-by: Inki Dae <inki.dae@samsung.com>