This was broken by https://github.com/google/ExoPlayer/commit/74a9d8f680995f2096c59fde6cd1ef6e85bb4d55
because DashManifestParser switched to setting Format.sampleMimeType to
text/vtt while SubtitleDecoderFactory was still expecting
application/x-mp4-vtt. This change teaches SubtitleDecoderFactory to
check both Format.containerMimeType and Format.sampleMimeType.

I'll investigate a follow-up change to remove
MimeTypes.APPLICATION_MP4VTT completely (it's currently still used in
AtomParsers).

Issue: #7985
PiperOrigin-RevId: 334771672