Multi user features :

->Use Usr App related fonctions instead of Global App related Functions

Signed-off-by: Sabera Djelti (sdi2) <sabera.djelti@open.eurogiciel.org>
@rakuco : This define will be replaced by a function that return an uid_t.

This define exists also at platform level.
By convenience, it will be better to keep it in this state to facilitate the changes when we will add the function that return the global user tz_platform_getGlobalUser()

Is possible to merge it even if there is a define?