I/O scheduler settings

(System settings > Performance > I/O scheduler)

This needs a change of permissions of /sys/block/mmcblk0/queue/scheduler,
analog to /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor.
(separate commit: http://review.cyanogenmod.org/#/c/32148/)

Change-Id: If24d9b9cedd26f5d4c0d4b6d591163e7333a7aa4