Use the proper memcg directory for the v2 hierarchy

When using the v1 hierarchy per process memcg directories exist under
/dev/memcg/apps. When using the v2 hierarchy per process memcg
directories exist under /sys/fs/cgroup. Hence this patch that selects
the proper top-level directory depending on the memcg version.

Bug: 213617178
Test: Verified Android operation inside the Cuttlefish emulator.
Change-Id: I7373fb407cb6ad2b1181579691ff54886fd36c24
Signed-off-by: Bart Van Assche <bvanassche@google.com>