With fMP4 implementation there will be two writers `DefaultMp4Writer`
and `FragmentedMp4Writer`.
Changes includes:
1. Make Mp4Writer as an abstract class and keep only common functionality
into it.
2. Create a DefaultMp4Writer which contains existing logic to write MP4.
3. The fMP4 logic needs to access `pending sample buffer info` at various
places, so did refactoring to split List<Pair<BufferInfo, ByteBuffer>>
into two separate lists.
PiperOrigin-RevId: 592861398
60c8273521