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/audio_processing_unittest.cc

@@ -1284,7 +1284,7 @@ TEST_F(ApmTest, AgcOnlyAdaptsWhenTargetSignalIsPresent) {
   geometry.push_back(webrtc::Point(0.05f, 0.f, 0.f));
   config.Set<Beamforming>(new Beamforming(true, geometry));
   testing::NiceMock<MockNonlinearBeamformer>* beamformer =
-      new testing::NiceMock<MockNonlinearBeamformer>(geometry);
+      new testing::NiceMock<MockNonlinearBeamformer>(geometry, 1u);
   std::unique_ptr<AudioProcessing> apm(
       AudioProcessing::Create(config, beamformer));
   EXPECT_EQ(kNoErr, apm->gain_control()->Enable(true));