Fix the cgroup v2 thread path

The name for the file with thread IDs is "tasks" in the v1 hierarchy and
"cgroup.threads" in the v2 hierarchy.

References:
* https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v1/cgroups.html
* https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html



Bug: 213617178
Fixes: 82b72a56 ("libprocessgroup: Add support for task profiles")
Change-Id: I87d67edeb12803cb83486be8f1b1b56a3f275a9d
Signed-off-by: Bart Van Assche <bvanassche@google.com>