SDK / exoplayer

- Rather than returning a map, return a DrmInitData object,
  with mapped and non-mapped implementations.
- Include a suitable mimeType to pass to the MediaDrm. Previously
  we were incorrectly passing the mimeType of the samples,
  where-as MediaDrm expects the container mimeType. Note that
  it doesn't matter whether the mimeType starts with "video" or
  "audio", hence using video mimeTypes everywhere.