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).
- This change is being added to avoid unnecessary creation of
  two extractors thereby improving the startup latency.

CRs-Fixed: 462087
Change-Id: Ia87eca73c4f81d37697fa85fd4f7c8cc8d406104