Fix PTREL-624

Fix PTREL-624

launchpad-preload@app.service set a wrong XDG_RUNTIME_DIR
environment.

We use new unit (user-session-launch@.service ) and symbol link
(user-session-launch@seat0-5000.service) to auto-trigger a user
session. This results in a environment
"XDG_RUNTIME_DIR=/run/user/5000" in the user session. But the
lauchpad service still use symbol link
"launchpad-preload@app.service" which result in a environment
"XDG_RUNTIME_DIR=/run/user/app". This make XDG_RUMTIME_DIR
dismatch between app launched by launchpad and program, e.g.
weston, launched in user session. And hence app launched by
launchpad failed at connecting socket /run/user/5000/wayland-0
that created by weston.

Change-Id: I7896dd001e9602772d96f4a20057cd90369a3a9f
Signed-off-by: Alex Wu <zhiwen.wu@linux.intel.com>