Pull out the PostFilter to its own NonlinearBeamformer API

This is done to avoid having a nonlinear component in the AEC path.
Now the linear delay and sum is run before the AEC and the postfilter after it.

This change landed originally at: https://codereview.webrtc.org/1982183002/

R=peah@webrtc.org
TBR=henrik.lundin@webrtc.org

Review URL: https://codereview.webrtc.org/2110593003 .

Cr-Commit-Position: refs/heads/master@{#13371}

webrtc/modules/audio_processing/include/audio_processing.h

@@ -31,8 +31,7 @@ struct AecCore;
 
 class AudioFrame;
 
-template<typename T>
-class Beamformer;
+class NonlinearBeamformer;
 
 class StreamConfig;
 class ProcessingConfig;
@@ -275,7 +274,7 @@ class AudioProcessing {
   static AudioProcessing* Create(const Config& config);
   // Only for testing.
   static AudioProcessing* Create(const Config& config,
-                                 Beamformer<float>* beamformer);
+                                 NonlinearBeamformer* beamformer);
   virtual ~AudioProcessing() {}
 
   // Initializes internal states, while retaining all user settings. This