libprocessgroup: restrict SetupCgroups to one-time usage and only by init

SetupCgroups is called by init process during early-init stage and is not
supposed to be called again by anyone else. Ensure that the caller is the
init process, make sure cgroup.rc file is written only one time, keep the
file descriptor to cgroup.rc file open by the init process to ensure all
its further mappings stay valid even if the file is deleted.

Bug: 124774415
Test: build, run, verify no errors or warning in the logcat

Change-Id: Ib8822cf0112db7744e28d442182d54dcf06f46f2
Signed-off-by: Suren Baghdasaryan <surenb@google.com>