libprocessgroup: switch freezer to cgroup v2

remove cgroup v1 freezer entries from init.rc, add a new cgroup v2
controller and modify plists to properly interact with it.

Bug: 154548692
Test: manually verified the the cgroup v1 freezer controller isn't
created and a new controller for cgroup v2 is created under the correct
sysfs directory.

Change-Id: I1b811300ade486f88fdbd157255a7f37750cc54d
Merged-In: I1b811300ade486f88fdbd157255a7f37750cc54d