Fix audio-video sync issue in DASH MP4 muxer

[Gautam-aman]

What is it?

• Bugfix (user facing)
• Feature (user facing)
• Codebase improvement (dev facing)
• Meta improvement to the project (dev facing)

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Description of the changes in your PR

Fix audio-video synchronization issue in DASH MP4 muxing.

Audio and video tracks extracted from DASH streams may have different starting
composition timestamps (PTS), which previously caused desynchronization.

This PR normalizes timestamps by:

• Capturing initial PTS for each track
• Computing a global minimum PTS once all tracks are initialized
• Introducing a stable per-track base timestamp (basePts)
• Preventing mid-stream timestamp reference switching
• Adjusting CTTS offset calculation accordingly

This ensures both tracks share a consistent timeline during muxing.

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APK testing

Tested using locally built APK.

• Verified on multiple YouTube videos (480p, 720p)
• Checked using frame-by-frame playback in VLC
• No audio-video desync observed

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Due diligence

• I read the contribution guidelines.
• The proposed changes follow the AI policy.
• I tested the changes using an emulator or a physical device.

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Issue

Fixes #12640

[absurdlylongusername]

Please do not remove the default PR template. Restore it and fill it in properly

[Gautam-aman]

Thanks! I’ve restored the PR template and filled it properly.

[absurdlylongusername]

Could you please give a detailed explanation of how your solution works and how it fixes the problem, as well as provide clear instructions of how to test that the solution fixes the problem?

Either the code must be self-explanatory, or it must be explained elsewhere in documentation such as in this PR description: but right now there is neither.

I don't know what PTS is or how this works, and it would be best for everyone to remove all assumed knowledge and have things be clear as possible and easily understood.

[TobiGr]

Thank you for the PR. I tried to run your changes, but they do not seem to fix the problem. I used the video from #12640 and still got a desync / no audio at 32:52. Please explain in detail how your fix works.

[TobiGr]

I don't understand your code here. Why do you introduce a temp var? Can't you set allSet = false directly? This should be equal to you code but is easier to understand:

Suggested change

	final boolean allSet;
	{
	boolean temp = true;
	for (int k = 0; k < readers.length; k++) {
	if (!firstPtsSet[k]) {
	temp = false;
	break;
	}
	}
	allSet = temp;
	}
	boolean allSet = true;
	for (int k = 0; allSet && k < readers.length; k++) {
	if (!firstPtsSet[k]) {
	allSet = false;
	}
	}

[TobiGr]

Why are these two vars arrays? They both only hold one value.