SDK / exoplayer

The MIME type is currently required to select a SubtitleDecoder
implementation in the TextRenderer. Future changes might remove this
requirement, so we pre-emptively mark the field as @Nullable.

The change in SingleSampleMediaSource ensures the track still maps to
the TextRenderer, otherwise it shows up as unmapped. Passing null MIME
type to MediaItem.Subtitle constructor now results in this from
EventLogger:

TextRenderer [
  Group:0, adaptive_supported=N/A [
    [ ] Track:0, id=null, mimeType=text/x-unknown, language=en, supported=NO_UNSUPPORTED_TYPE
  ]
]

PiperOrigin-RevId: 398010809