Modularize VoiceInteraction/SoundTrigger

Rather than building VoiceInteraction monolithically, split out
SoundTrigger.

This allows for better build performance, more localized testing,
specific build flags, etc.

Split out the middleware in a static lib for now, but don't expose it
as such, since it will be refactored into SoundTrigger soon.

Fixes: 277374624
Test: Compiles (build changes only)
Change-Id: I631d073d81e578c70b033d0a43543ef89dda9194