Media Grid: 

* Don't store multiple references to the Edit Attachments frame.
* Fix keyboard navigation event delegation and proxy.
* Remove redundant function calls.

See #24716.

Built from https://develop.svn.wordpress.org/trunk@29089


git-svn-id: http://core.svn.wordpress.org/trunk@28875 1a063a9b-81f0-0310-95a4-ce76da25c4cd