libstagefright: Move checks for creating new extractor to ExtendedExtractor

- Move all the checks and creation of the extended extractor
into ExtendedExtractor.
- Restrict creation of new extractor to the following conditions
  o default extractor is NULL
  o default extractor says the content is video only
    or has an unrecognized audio stream
  o the audio stream is a amr-wb (plus).
- Avoid creation of two extractor, this improve start-up latency.
CRs-Fixed: 507045

Change-Id: Id9f9428f0ca5781ecdbc9b27f6c6d930df820751