Migrate the blkio controller to the v2 cgroup hierarchy

There are multiple use cases in Android for which background writes need
to be controlled via the cgroup mechanism. The cgroup mechanism can only
control background writes if both the blkio and memcg controllers are
mounted in the v2 cgroup hierarchy. Hence this patch that migrates the
blkio controller from the v1 to the v2 cgroup hierarchy.

The changes compared to the previous version of this CL are as follows:
- The JoinCgroup actions for the "io" controller have been left out
  since these caused processes to be migrated to the v2 root cgroup.
- The BfqWeight / CfqGroupIdle / CfqWeight settings have been included
  in this CL instead of applying these settings as a separate CL.

Change-Id: I67e06ce3462bb1c1345dba78f8d3d655b6519c74
Signed-off-by: Bart Van Assche <bvanassche@google.com>