libprocessgroup: Rename files a_cgroup_controller.cpp, a_cgroup_file.cpp

These are for ACgroupController and ACgroupFile, but share a filename
with libcgrouprc_format's CgroupController which can be confusing.

Bug: 349105928
Test: m
Change-Id: Idb270068169b71b0a33101407ddbb5a38cc8d469