SDK / exoplayer

We stopped using using this MIME type in
https://github.com/google/ExoPlayer/commit/74a9d8f680995f2096c59fde6cd1ef6e85bb4d55

This broke subtitle decoding in some cases (Issue: #7985), which I
fixed in
https://github.com/google/ExoPlayer/commit/7b8895d655c9b48e042f66045bc9c7ab27e93346.

After some discussion we've decided SubtitleDecoderFactory shouldn't
depend on Format.containerMimeType (since the samples have already been
extracted by this point, so the container shouldn't matter). So this
change fixes DashManifestParser to use MimeTypes.APPLICATION_MP4VTT (and
reverts the no-longer-needed SubtitleDecoderFactory change).

PiperOrigin-RevId: 336668450