Let blkio cgroup follow sched_policy

This reverts commit e8678cf8.

With b/131214885 resolved, we should reland the change for bg sched group.
If a thread explicitly calls set_sched_policy to set a thread to background, it should not be latency sensitive.

This would help background I/O impacting foreground I/O e.g. b/65727988

Test: build and boot
Bug: 140209629
Change-Id: I218db084fb733cad57af8a5eae926818a6a2cdb8