Media: when activating a mode after [29056], don't trigger the `{mode}:activate` event twice in a row.

See #29366.

Built from https://develop.svn.wordpress.org/trunk@29601


git-svn-id: http://core.svn.wordpress.org/trunk@29375 1a063a9b-81f0-0310-95a4-ce76da25c4cd