Media Grid: Don't make checkboxes tabbable when in bulk select media grid mode, fixes tabbing in bulk select mode.

Props adamsilverstein.
See #28822.

Built from https://develop.svn.wordpress.org/trunk@29585


git-svn-id: http://core.svn.wordpress.org/trunk@29359 1a063a9b-81f0-0310-95a4-ce76da25c4cd