fix: seek during startup on low power devices

[grabofus]

This PR will...

Fix an edge-case on webOS 5 (and possible other low power devices), where synchronizeToLiveEdge() could seek forward while the first segment was being appended to the buffer.

Why is this Pull Request needed?

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

Resolves issues:

#6998

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

[robwalch]

Suggested change

	const mediaCurrentTime = media.currentTime;
	const currentTime =
	this.hasEnoughToStart || mediaCurrentTime > 0
	? mediaCurrentTime
	: this.startPosition;
	const currentTime = this.getLoadPosition();

[robwalch]

Use getLoadPosition to determine the stream-controller's target position. This will return currentTime when appropriate (even when its value is 0) or nextLoadPosition prior to start and when detached, which unlike startPosition will advance as needed.