SSA-in-MKV doesn't support simultaneous subtitles

### ExoPlayer Version

2.17.1

### Devices that reproduce the issue

- All devices I used. It's not device-relevant.
- Devices I used: OnePlus 8T, OnePlus 10 Pro, Xiaomi Mi11 Ultra, Asus ROG Phone 4, Red Magic 6, Xiaomi Redmi Note 8 Pro, Samsung Galaxy S21

- All of the aforementioned devices run Android 10 to 12.

### Devices that do not reproduce the issue

_No response_

### Reproducible in the demo app?

Yes

### Reproduction steps

- Loading any media file.
- Passing an SRT/ASS subtitle file to ExoPlayer that contains two (or more) lines that show up at the same time.

### Expected result

- Both subtitles should appear at the same time.

### Actual result

- If two subtitles are overlapping, the subtitle that appears later will remove the existence of the subtitle that appeared before it, albeit both co-existing during that timeframe.
- In many movies and series, subtitles overlap in some dialogues.

### Media

[OverlappingSubsDemo.zip](https://github.com/google/ExoPlayer/files/8779026/OverlappingSubsDemo.zip)

As for the video file, just grab any media from https://thedigitaltheater.com/dts-trailers/

### Bug Report

- [ ] You will email the zip file produced by `adb bugreport` to dev.exoplayer@gmail.com after filing this issue.