CachedAppOptimizer: use new cgroup api for freezer path

Use the new cgroup API in place of the hardcoded path.
The new libprocessgroup API is defined in https://r.android.com/1607196

Bug: 180056337
Test: verified the freezer work via logs
Change-Id: I73c0976f3a917890bcd888c232975e2258f87b5c