Fix fmp4 segmented audio group switching

[robwalch]

This PR will...

• Reset transmuxer on audio-tracks-updated to handle audio group track switching
• Added groupId to AudioTrackSwitchingData and SubtitleTrackSwitchData (just added name recently and in light of this issue, groupId is also something you'd want to know about an upcoming track change)

Why is this Pull Request needed?

The transmuxer doesn't see audio groups so tracks in different groups with the same index appear as contiguous segments. Resetting the transmuxer on audio track update deals with this type of discontinuity resetting where audio is appended when the active group is changed.

hls.js/src/demux/transmuxer-interface.ts

Lines 159 to 163 in 072ef4b

	const trackSwitch = !(lastFrag && chunkMeta.level === lastFrag.level);
	const snDiff = lastFrag ? chunkMeta.sn - (lastFrag.sn as number) : -1;
	const partDiff = this.part ? chunkMeta.part - this.part.index : 1;
	const contiguous =
	!trackSwitch && (snDiff === 1 || (snDiff === 0 && partDiff === 1));

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

I considered adding groupId or original track id indexes (across groups) to the transmux pipeline but that would involve adding them to Fragment objects, ChunkMeta, multiple playlist/track types, as well multiple event types. This change has a much smaller footprint, making it a safer patch.

Resolves issues:

Resolves #3741

Checklist

• changes have been done against master branch, and PR does not conflict
• API or design changes are documented in API.md