AudioCodec: rename getSampleFormat to getAVSampleFormat
AudioFrame: add getSampleFormat to get it as string

Also rename getSampleFormat to getAVSampleFormat in this class.
GeneratorAudio: rename _audioDesc to _codec

* According to the class name "AudioCodec".
* "getAudioCodec" returns a reference instead of make a copy.
* Rename "setAudioDesc" to "setAudioCodec".
GeneratorVideo: rename _videoDesc to _codec

* According to the class name "VideoCodec".
* "getVideoCodec" returns a reference instead of make a copy.
* Rename "setVideoDesc" to "setVideoCodec".