CachedAppOptimizer: enable single cgroup freezer

The cgroup v2 uid/pid hierarchy was reverted because of
http://b/179006802 leaving the underlying freezer implementation to a
single cgroup. Enable the freezer at boot until the new hierarchy can be
brought back.

Bug: 179006802
Test: verified that the freezer is enabled at boot
Change-Id: I88041a3efb06724e4bc2d78fea017778c365d387