fix(latency-controller): only sync live stream

[FredZeng]

This PR will...

maxLiveSyncPlaybackRate should only work for live stream.

I'm using same config ({ maxLiveSyncPlaybackRate: 1.2 }) to play normal playback and live stream.
When playing playback, we hope users can change playback rate as needed. However, things work not quiet well.

When user changes playback rate to 2, the player will reset to 1 quickly.

To figure out what happen, I have debugged the latency-controller.ts file.
It seems that we want to listen 'timeupdate' event when receive MEDIA_ATTACHED(https://github.com/video-dev/hls.js/blob/master/src/controller/latency-controller.ts#L153), and remove 'timeupdate' event when receive LEVEL_UPDATED(https://github.com/video-dev/hls.js/blob/master/src/controller/latency-controller.ts#L177).

Since the MEDIA_ATTACHED is triggered by _onMediaSourceOpen fired by sourceopen event, the order between MEDIA_ATTACHED and LEVEL_UPDATED cannot be guaranteed.

When LEVEL_UPDATED triggered before MEDIA_ATTACHED, the latency tracer (mouted by 'timeupdate') will never get a chance to stop, making user unable to change playback rate.

Why is this Pull Request needed?

To make sure maxLiveSyncPlaybackRate config only work for live stream.

Are there any points in the code the reviewer needs to double check?

Resolves issues:

Checklist

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md