Let blkio cgroup follow sched_policy

This reverts commit e8678cf883ee78ab9fe5ea47e6d9b24ef7c8d4f7.

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
1 file changed