Fix a number of issues with talkie playback.

Summary:
This fixes a number of issues with taklie playback. Specifically:

- Scrubbing flashed hotnumber changes aggressively.
- Hitting play after seek:
  - The code reset back to its original state
  - The canvas drawing would get erased
- During seek:
  - Drawing would constantly be overwritten (line gets thicker)

Remove some test code.

Test Plan: I've added in a new audio demo that we can use to manually test playback. Attempt to play through the talkie and manually scrub around, making sure everything is working as expected.

Reviewers: brian, pamela

Reviewed By: pamela

Subscribers: alexrodrigues, #cs

Differential Revision: http://phabricator.khanacademy.org/D13672