MKAudioInput: set MKAudioDevice input block after calling initializeMixer.

This is obviously racy. If our block is called before our call to
initializeMixer is done, it's accessing uninitialized memory.
MKAudioInput: fix mic boost memory corruption when resampling.
MKAudioInput: correctly preprocess resampled frames.
MKAudioInput: fix peakSignal miscalculation when resampling.