Fix start-time functionality in the video player.

**Description**

When there is a start-time:

- Initially, the video starts playing from the start-time.
- Whenever the player resets for any reason (such as reaching the end of the video), it will reset to start-time instead of 00:00:00.
- It should not be the case that any time the user presses play, it start playing from start-time.

**Jira Issue**

[BLD-659](https://edx-wiki.atlassian.net/browse/BLD-659)

**Reviewers**

- @jmclaus 
- @wedaly