[tmpfs] Write files under XDG_RUNTIME_DIR instead of HOME. Contributes to NEMO#489.

XDG_RUNTIME_DIR is tmpfs where HOME is just normal filesystem. We can avoid
some write operations to emmc when we do move these to tmpfs. Also
these files are something that should be reset on each boot anyway.

Signed-off-by: Marko Saukko <marko.saukko@jollamobile.com>