FFmpegExtractor: set proper mimetype for VP8/9

This allows other stagefright codecs to be used even when the extended
extractor is used. (Right now, if the the extended extractor is forced
to be used with VP8/9, then 'vtrial' mimetype will be used.)

Change-Id: I7af0bf609d07871b8956d7799cc036c0b8d18d37