Cleanup security context after application termination.

Cleanup security context after application termination.

security_manager_prepare_app() and security_manager_cleanup_app()
are a pair of functions to setup/clean security context of application.

Each application is launched in a separate mount namespace and
linked to this namespace.
Such design allows to change privacy privilege status at app runtime.
After application termination this link should be removed.

Change-Id: I96bb6b88d3822d736e2985c750ec5697165abef2
Signed-off-by: Hwankyu Jhun <h.jhun@samsung.com>