SDK / exoplayer

1. Rename "extensions" package to "decoder". It's used by both
   text and (should be used by) metadata, so it's no longer just
   for extensions.
2. Move Buffer objects move into the decoder package.
3. Rename SubtitleParser and MetadataParser to SubtitleDecoder
   and MetadataDecoder respectively, since they extend Decoder.
   Ditto for all subclasses.
4. Subtitle and Metadata decoders now throw their own exception
   types rather than ParserException.
5. Move MediaCodec classes into a mediacodec package, with the
   exception of the concrete audio and video renderers.
6. Create an audio package to hold the two audio renderer classes
   plus related util classes.
7. Create a video package to hold the one video renderer class
   plus related util classes.

After this change the following nice properties hold:

1. Want a video renderer? Look in the video package. Ditto for
   audio, text and metadata.
2. All TrackRenderer implementations use a decoder of some kind
   to decode buffers received from the source, so we have
   consistent terminology there.
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