Revert "Revert "libprocessgroup: move freezer to uid/pid hierarchy""

This reverts commit b01aa994.

Reason for revert: blocking issue was resolved https://partner-android-review.googlesource.com/c/kernel/private/gs-google/+/1846507

Change-Id: If125fe9af53e2b519955f021208d5d5530c6277e