Fixes T3359 - Tooltips stop receiving updates after talk-through ends

Summary:
When soundManager finished playing it used to just trigger the "playPaused" and "playEnded" events,
         which was enough to update the buttons, but did not actually change state inside record.js
         so record.js still thought it was playing. To fix this I now call the actualy stopPlayback()
         which alters the internal state of record.js

Test Plan:
- Watch talkthrough to the end
- Type "fill(" and see tooltip autocomplete
- Use number scrubber and see values change

Reviewers: john, pamela

Reviewed By: pamela

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D14555