Move wp_plupload_default_settings() to the admin_enqueue_scripts hook to...

Move wp_plupload_default_settings() to the admin_enqueue_scripts hook to prevent fatal errors when admin_init is run outside of normal admin-header execution, like admin-ajax. see #19910. fixes #20240.

git-svn-id: https://core.svn.wordpress.org/trunk@20189 1a063a9b-81f0-0310-95a4-ce76da25c4cd