Encapsulate Checkpoint internal state

[bg-chun]

This PR encapsulates the internal state of Checkpoint and prevents direct access to CheckpointV1 from DeviceState.

[bg-chun]

commits has been squashed

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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/lifecycle stale

[k8s-triage-robot]

The Kubernetes project currently lacks enough active contributors to adequately respond to all PRs.

This bot triages PRs according to the following rules:

• After 90d of inactivity, lifecycle/stale is applied
• After 30d of inactivity since lifecycle/stale was applied, lifecycle/rotten is applied
• After 30d of inactivity since lifecycle/rotten was applied, the PR is closed

You can:

• Mark this PR as fresh with /remove-lifecycle rotten
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• Offer to help out with Issue Triage

Please send feedback to sig-contributor-experience at kubernetes/community.

/lifecycle rotten