demux: mp4: fix chapters showing

#25561 (closed) was marked as fixed by !3564 (merged), for me however subtitles are still showing up. The changes in this commit resolve that issue. Please note that I am explicitly not familiar with VLC or the MP4 format in the slightest, and this was mostly just me experimenting. I do not know if switching from UNKNOWN_ES to SPU_ES might break with other chapter formats or something along those lines.