Optimize audio context length for short recordings

[nchudleigh]

I added logic to dynamically adjust audio_ctx based on the input audio length.
Gives better performance with no degradation of WER in my testing.

File	Duration	Opt Encode (ms)	Base Encode (ms)	Improvement
jfk.wav	~11s	~13.5	~19.7	~31%
en-1-16khz.wav	~20s	~148.6	~155.4	~4.4%
en-2-16khz.wav	~30s	~21.0	~20.2	~0% (Noise)
en-2.wav	~30s	~21.1	~21.1	~0% (Noise)