FFmpegExtractor: add mp3 to supported list

This prevents mp3 files from being sniffed twice everytime.

(BetterSniffFFMPEG -> fail -> LegacySniffFFMPEG -> fail again)

Change-Id: I606cc430089d36f8cd4627d689e095087605373b
FFmpegExtractor: mark VP9 as supported

Software codec for VP9 is provided by Google

Change-Id: I9383924ba3f0efe823a757e560db0105925f7ca6
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
FFmpegExtractor: don't print list of supported formats

It's just log-spam and doesn't aid debugging in any way
(the value is always constant)

Change-Id: I06bd88675c8cd306523227cde8f2744077f3783d