[DO NOT MERGE] 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 blkio controller has been marked as optional since not all Android
kernels enable this controller (CONFIG_BLK_CGROUP).

This patch increases the TOTAL_BOOT_TIME for devices with a 4.19 kernel
(redfin) from 18.9 s to 20 s. This patch does not affect the boot time
for devices with a 5.10 or 5.15 kernel.

This patch increases the time spent in CgroupMap::ActivateControllers()
by 25 microseconds in Cuttlefish on an x86-64 CPU.
CgroupMap::ActivateControllers() is called by Service::Start().

Bug: 213617178
Test: Cuttlefish and various phones
Change-Id: I3c07c1be84c3feb277b7d7003652d5d3b57c6541
Signed-off-by: Bart Van Assche <bvanassche@google.com>