atrace: Enable cgroup tracing

In prep for work involving analyzing cgroup CPU time distribution,
enable this trace category in systrace.

Bug: 37997910
Test: Enabled cgroup category by systrace as root/non-root

Change-Id: Ie16cc347c3399eedcd7604fc3e9af607a45f34dc
Signed-off-by: Joel Fernandes <joelaf@google.com>