Automatic merge from submit-queue (batch tested with PRs 58422, 58229, 58421, 58435, 58475). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

kubelet: imagegc: exempt sandbox image

The image GC logic currently does not consider the sandbox image to be in-use by pods, since it isn't explicitly listed in the pod spec.  However, it is trivially in-use if there are any pods running on the node.

This change adds logic to exempt the sandbox image from GC by always considering it as in-use.

**Reviewer Note**
I am changing `(m *kubeGenericRuntimeManager) GetImageRef` to return the ID always rather than the first tag if it exists.  Seemed ok to me.  Makes some error messages a little less readable in that the ID will be printed and not the tag.  Just wanted to see what reviewers think about this.

@derekwaynecarr @dashpole