Remove RefBase from the extractor API

- Add MetaDataBase base class that MetaData derives from, but which
  does not derive from RefBase.
- MediaBuffer::meta_data() now returns a MetaDataBase& rather than an
  sp<MetaData>
- Rename MediaSourceBase to MediaTrack.
- MediaSource no longer derives from MediaSourceBase (or MediaTrack)
- MediaTrack::getFormat(), MediaExtractor::getTrackMetaData() and
  MediaExtractor::getMetaData() all take a MetaDataBase& parameter that
  they fill out, rather than returning a MetaData directly (the
  corresponding methods on MediaSource and RemoteMediaExtractor continue
  to return MetaData)

Bug: 67908544
Test: CTS MediaPlayerTest, DecoderTest, EncodeDecodeTest, manually record video

Change-Id: Ib531ab309061290be33d40d6100c9a8127e22083