crane: Add timestamp to flatten layer

[nivbend]

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.

Set both fields to the current time.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[nivbend]

Fixes #1996.