Customize: Prevent re-importing starter content when changeset is saved as draft or scheduled.

Themes cannot currently be switched in Customizer after changeset is saved anyway.

Props dlh, westonruter.
See #40146, #42411, #42126.
Fixes #42395.

Built from https://develop.svn.wordpress.org/trunk@42107


git-svn-id: https://core.svn.wordpress.org/trunk@41936 1a063a9b-81f0-0310-95a4-ce76da25c4cd