freezer: switch to cgroup v2 freezer

cgroup v2 freezer has a single hierarchy and is mounted under
/sys/fs/cgroup. Proper v2 freezer support in a system can be checked by
verifying that the file "cgroup.freeze" is present in a freezer subgroup.

Bug: 154548692
Test: manually verified that processes are frozen and unfrozen

Change-Id: Ib966a957490ec986eb14aba6492832c96e147896
Merged-In: Ib966a957490ec986eb14aba6492832c96e147896