libprocessgroup: adopt cgroup v2 freezer search path

With the new uid/pid hierarchy the search path looks into uid_0, which
is always present.

Bug: 168907513
Test: verified that the cgroup v2 hierarchy works correctly in normal
mode
Test: verified that the cgroup v2 hierarchy works correctly in recovery
mode

Change-Id: If33594e4891128148da64bed47e68fa74667013d