libprocessgroup: uid/pid hierarchy for cgroup v2

Enable the uid/pid hierarchy for all groups when using cgroup v2. Mount
the hierarchy under the cgroup v2 root. Make sure that all files under
the hierarchy are accessible by the system user.

Test: booted the device, tested the freezer cgroup, manually verified
the working of the freezer from logs and by checking statuses of
processes.

Bug: 168907513
Test: Booted the device, verified no regressions on process group access
Change-Id: I73f3e767d377902af6e12facb503b9136fb39e08