sepolicy: allow system_server to write to cgroup_v2

During boot, system_server will need to write to files under
/sys/fs/cgroup/freezer. Change the cgroup_v2 policy to allow this
operation.

Test: booted device with change, verified that files are properly
accessed.
Bug: 154548692

Change-Id: I2ccc112c8870129cb1b8312023b54268312efcca
1 file changed