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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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=127326805
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