memcg: add permission check

Use the 'allow_attach' handler for the 'mem' cgroup to allow
non-root processes to add arbitrary processes to a 'mem' cgroup
if it has the CAP_SYS_NICE capability set.

Bug: 18260435
Change-Id: If7d37bf90c1544024c4db53351adba6a64966250
Signed-off-by: Rom Lemarchand <romlem@android.com>