block: Disable I/O stats accounting by default

While Android userspace (e.g. storaged) does use iostats via
/proc/diskstats, init will explicitly enable iostats for the devices on
which it is primarily used - sda and sdf. Avoid the 0.5-1% overhead for
block devices that do not need it.

Signed-off-by: kdrag0n <dragon@khronodragon.com>
Signed-off-by: Lau <laststandrighthere@gmail.com>