Replace nsAudioStream::Format with an AUDIO_OUTPUT_FORMAT enum value so we
can use it as a template parameter.
Introduce AudioSampleTraits<AudioSampleFormat> to give us access to the C++ type
corresponding to an enum value.
Move SampleToFloat/FloatToSample to AudioSampleFormat.h.
Introduce ConvertAudioSamples and ConvertAudioSamplesWithScale functions
and use them from various places.
Moves AudioDataValue to AudioSampleFormat.h. The name isn't great, but it'll do.
