Fix hardcoded library path in xwalk script and Service dbus File

The binary xwalk is stored in 32 bit arch platform in /usr/lib/xwalk/xwalk
and for X64 arch in /usr/lib64/xwalk/xwalk

Xe should adapt the script file xwalk and dbus service following architecture
DBUS deamon can't spawn the xwalk process automatically due to wrong path

Signed-off-by: Baptiste DURAND <baptiste.durand@open.eurogiciel.org>