XML-RPC featured image and media preparation cleanup.

* Introduce _prepare_media_item().
* Use it to prepare post_thumbnail info in _prepare_post().
* Also use _prepare_media_item() in wp_getMediaLibrary() and wp_getMediaItem() so that all media is prepared consistently and uses the same filters.

Props maxcutler
Fixes #20409


git-svn-id: https://core.svn.wordpress.org/trunk@20608 1a063a9b-81f0-0310-95a4-ce76da25c4cd