Media Grid: don't blow away the Attachment Details modal every time a new model is loaded.

Props ericlewis.
Fixes #29104.

Built from https://develop.svn.wordpress.org/trunk@29368


git-svn-id: http://core.svn.wordpress.org/trunk@29146 1a063a9b-81f0-0310-95a4-ce76da25c4cd