This adds a generic interface for an echo detector, and makes it possible to inject one into the audio processing module. Bug: webrtc:8732 Change-Id: I30d97aeb829307b2ae9c4dbeb9a3e15ab7ec0912 Reviewed-on: https://webrtc-review.googlesource.com/38900 Commit-Queue: Ivo Creusen <ivoc@webrtc.org> Reviewed-by: Per Åhgren <peah@webrtc.org> Cr-Commit-Position: refs/heads/master@{#21588}