This was missed in da724c8cc4
I tried to write a test for this, but got stuck crafting valid test
data. I was able to create a new fragmented MP4 file containing only a
TTML track:
```shell
$ MP4Box -add simple.ttml -frag 2000 sample_fragmented_ttml.mp4
```
Then I tried naively removing the `ftyp` and `moov` boxes with a hex
editor, but using this in `FragmentedMp4ExtractorNoSniffingTest` gave
me an `EOFException` that I didn't get to the root cause of.
Issue: androidx/media#1779
#cherrypick
PiperOrigin-RevId: 683667850
546d7da2f2