Provide a way to set the stream start time in the manifestparsed event handler

[caridley]

Description

Implements #3434

Added method Player.setStartTime(startTime: number) which can be used to update the stream start position in the manifestparsed event handler based on information obtained from the manifest, such as the position and duration of ad breaks. This is done in a way that avoids unnecessary fetching of segments at the beginning of the stream prior to fetching segments at the desired start position.

This change reduces our VOD DAI stream start times from ~4.5 to ~3.5 seconds, which has a measurable effect on our QOS metrics.

Note that I tried to implement this by setting the value of player.getMediaElement().currentTime in the manifestparsed event handler, but that did not work with Safari.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• This change requires a documentation update - new function Player.setStartTime(startTime:number)

Checklist:

• I have signed the Google CLA https://cla.developers.google.com
• My code follows the style guidelines of this project
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have verified my change on multiple browsers on different platforms
• I have run ./build/all.py and the build passes
• I have run ./build/test.py and all tests pass

[caridley]

@joeyparrish Hoping to get a code review on this. It is an alternative implementation of #3485 which you reviewed last week.

[theodab]

Sorry for the delay! I forgot to hit the "submit review" button last week.

[shaka-bot]

All tests passed!