Improve interleaving of processed frames

Nobody has claimed this yet.

**Problem**
From my reading of the code, and logging the PTS values of samples fed into the `MediaTarget`, it appears that samples are fetch from each track sequentially (round robin). This results in a stream similar to "A/V/A/V/A/V... etc". However, this doesn't take into account that the duration of each frame (aka sample) is likely to be very different. For a video that has a frame rate of 30fps, a single video frame will likely be presented while multiple audio frames are rendered via the AudioTrack.

This behaviour results in the audio and video samples that should be presented at the same time being sparsely spread throughout the output file. The default Android muxer (or other open source muxers) will be able to fix the interleaving. For small files, this is likely to not be that noticeable, but would require those players to seek back and forth within the file. The issue will become more noticeable as the output file grows (it's worth noting that ExoPlayer has different buffering logic per track compared to other video players). If we wanted to support a segmented output file, this behaviour is also problematic. We need each segment to contain the same duration for every contained stream.

**Proposed Solution**
One possible solution could be:
- Modify `TrackTranscoder` to report the PTS of the last frame processed
- Modify `TransformationJob.processNextFrame` to be smarter about which `TrackTranscoder` to process next. We could define a max duration that streams are allowed to written ahead. If we observe that a stream (e.g. video) is now beyond that, we will continue to process the Audio track until it's caught up.

I took a look at Google's Media3 / Transformer and looks like they're doing something very similar: https://github.com/androidx/media/blob/main/libraries/transformer/src/main/java/androidx/media3/transformer/MuxerWrapper.java#L57

Start by reading TrackTranscoder and TransformationJob.processNextFrame to confirm the current round-robin selection and identify how processed-frame PTS values can be exposed. Compare the proposed approach with Media3's MuxerWrapper, then define completion as interleaving samples by timestamp so tracks do not run substantially ahead and segments contain comparable durations.

Written by the indexing model from the issue text.