libprocessgroup: Reject JoinCgroup actions for v2 controllers

A JoinCgroup action for a v2 cgroup controller migrates a process or task
from the uid_%d/pid_%d cgroup into another cgroup, e.g. the root cgroup.
This may make services unkillable because Service::Stop() only stops a
service if the uid_%d/pid_%d cgroup still exists when Service::Stop() is
called.

Bug: 309674654
Change-Id: I20b797afdf596125ff5a6ed41cb33fe59b84ac88
Signed-off-by: Bart Van Assche <bvanassche@google.com>