container: Only umount /sys/fs/cgroup/schedtune if needed

Silences the following useless log from "waydroid log" outputs when the
directory either doesn't exist at all (mainline Linux devices), or when
it's not mounted on Halium:

  (027693) [15:21:39] % umount -l /sys/fs/cgroup/schedtune
  umount: /sys/fs/cgroup/schedtune: not mounted

Reference: https://docs.python.org/3/library/os.path.html#os.path.ismount