Allow apexd to write to sysfs loop device parameters.

To configure read-ahead on loop devices, eg.
/sys/devices/virtual/block/loop0/queue/read_ahead_kb

Bug: 120776455
Test: configuring read-ahead on loop devices works from apexd
Change-Id: Ib25372358e8ca62fa634daf286e4b64e635fac58
7 files changed