Set the I/O scheduler of loop devices to 'none'

The user interface of Android devices becomes very slow under memory
pressure. This is because Android uses the zram driver on top of the loop
driver for swapping, because under memory pressure the swap code alternates
reads and writes quickly, because mq-deadline is the default scheduler for
loop devices and because mq-deadline delays writes by five seconds for such
a workload with default settings. Fix this by selecting I/O scheduler 'none'
for loop devices.

Bug: 194450129
Test: Built Android images, installed these and verified that the I/O scheduler of all loop devices is 'none' instead of 'mq-deadline'.
Change-Id: Ia5f606504b663948ab56955cad5a71885a356430
Signed-off-by: Bart Van Assche <bvanassche@acm.org>