SDK / exoplayer

In this CL:

* PesReader moves out of TsExtractor and becomes public.
* ElementaryStreamReaderFactory becomes TsPayloadReaderFactory and is
    moved to TsPayloadReader.
* The TsPayloadReaderFactory is in charge of wrapping any
    ElementaryStreamReaders with a PesReader.

Incoming:
* Extract SectionReader supperclass (analog to PesReader, but for
    sections) from Pat and Pmt readers.
* Add a ScteReader, wrapped by a section reader, and include it in
    the DefaultTsPayloadReaderFactory.

Issue:#726

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=136707706