Fix crash in getting system keyboard layout

In get_system_default_layout(), use a static variable for
xkl_engine_get_instance() result, and don't close the X Display.

This fixes the crash that happens at the second call of
get_system_default_layout(): xkl_engine_get_instance() returns a singleton
which saves the passed X Display instance, so we must never close it.

https://launchpad.net/bugs/505972