Customize: Ensure `autosaving` message gets sent to preview after initial...

Customize: Ensure `autosaving` message gets sent to preview after initial change when `saved` state is first dirty.

Amends [41667].
Fixes #42475 for trunk.

Built from https://develop.svn.wordpress.org/trunk@42136


git-svn-id: https://core.svn.wordpress.org/trunk@41967 1a063a9b-81f0-0310-95a4-ce76da25c4cd