rkt: Implement pod FinishedAt

This is implemented via touching a file on stop as a hook in the systemd
unit. The ctime of this file is then used to get the `finishedAt` time
in the future.
In addition, this changes the `startedAt` and `createdAt` to use the api
server's results rather than the annotations it previously used.

It's possible we might want to move this into the api in the future.

Fixes #23887

pkg/kubelet/container/os.go

@@ -25,6 +25,7 @@ import (
 type OSInterface interface {
 	Mkdir(path string, perm os.FileMode) error
 	Symlink(oldname string, newname string) error
+	Stat(path string) (os.FileInfo, error)
 }
 
 // RealOS is used to dispatch the real system level operaitons.
@@ -39,3 +40,8 @@ func (RealOS) Mkdir(path string, perm os.FileMode) error {
 func (RealOS) Symlink(oldname string, newname string) error {
 	return os.Symlink(oldname, newname)
 }
+
+// Stat will call os.Stat to get the FileInfo for a given path
+func (RealOS) Stat(path string) (os.FileInfo, error) {
+	return os.Stat(path)
+}