Set idle-time to 0 in weston.service.

As described weston.ini.man, "idle-time" should be set 0 if user
want not to enable screen saver.

A value of 0 disables the timeout.

Change-Id: I17cff212a5231be05008ac1a5acc23d86bb9be51
Signed-off-by: Harunobu Kurokawa <harunobu.kurokawa.dn@renesas.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/9693
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>