Refactoring done to more easily and cleanly add SIMD optimizations and to remove `GatedRecurrentLayer` from the RNN VAD api. Bug: webrtc:10480 Change-Id: Ie1dffdd9b19c57c03a0b634f6818c0780456a66c Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/195445 Commit-Queue: Alessio Bazzica <alessiob@webrtc.org> Reviewed-by: Jakob Ivarsson <jakobi@webrtc.org> Cr-Commit-Position: refs/heads/master@{#32770}