crane: Add timestamp to flatten layer

[Stephanie0829]

Fixes #2115, which was reopened from #1974.

When flattening an image the blob's created field is the same as the original image's, while the flattened layer's created field is a zero timestamp. This makes it impossible to figure out when was the flattened image actually created and distinguish it from the original.

After using crane flatten, the new layer does not have created time set. This will set the created time to the current time.

Tested:

./hack/presubmit.sh 

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[Stephanie0829]

Hi! Created this PR for visibility since the previous PR was labeled as stale. @jonjohnsonjr @Subserial @imjasonh @brandonmichigangithub Mind reviewing this so these changes can be merged (much appreciated)?