libprocessgroup: support for cgroup v2 hierarchy

for a first implementation the cgroup v2 freezer controller will be used in a
way similar to cgroup v1, that is a single child group will hold all frozen
processes. Some adjustments are needed for the new structure.

- Add support for cgroup v2 syntax under procfs.
- Separate creation of a directory with ownership/mode changes to allow changes
    after mounting the cgroup kernfs root.
- Allow the creation of sub-groups under a cgroup v2 hierarchy.

Bug: 154548692
Test: manually verified that a proper cgroup v2 hierarchy is created and
accessible

Change-Id: I9af59e8214acaead3f520a94c95e75394c0df948
Merged-In: I9af59e8214acaead3f520a94c95e75394c0df948