Themes: Switch back from throttling to debouncing in theme searches on admin screen.

Start debouncing after initial search performed when `search` query param is present to prevent initial "flash of unsearched themes".

Props afercia, westonruter.
Amends [41797].
See #40254.
Fixes #42348.

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git-svn-id: https://core.svn.wordpress.org/trunk@41863 1a063a9b-81f0-0310-95a4-ce76da25c4cd