When creating audio and video MCE views, listen to the players within each iframe to capture the "play" event. When a player plays, pause the players in every other iframe sandbox.

Fixes #29384.

Built from https://develop.svn.wordpress.org/trunk@30642


git-svn-id: http://core.svn.wordpress.org/trunk@30632 1a063a9b-81f0-0310-95a4-ce76da25c4cd