Restore I/O priority support

CL aosp/2929791 removed I/O priority support to prepare for a clean
revert of the CL that migrates the blkio controller from the v1 to the
v2 cgroup hierarchy. Since there was no other reason to revert the I/O
priority CL, restore I/O priority support.

Bug: 186902601
Change-Id: I1a4053140ab55973878bfeacfb546da3c601a895
Signed-off-by: Bart Van Assche <bvanassche@google.com>