Add core multi-channel pipeline in AEC3
This CL adds basic the basic pipeline to support multi-channel
processing in AEC3.

Apart from that, it removes the 8 kHz processing support in several
places of the AEC3 code.

Bug: webrtc:10913
Change-Id: If5b75fa325ed0071deea94a7546cb4a7adf22137
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/150332
Commit-Queue: Per Åhgren <peah@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29017}
diff --git a/modules/audio_processing/audio_processing_impl.cc b/modules/audio_processing/audio_processing_impl.cc
index bc61b52..d639fd5 100644
--- a/modules/audio_processing/audio_processing_impl.cc
+++ b/modules/audio_processing/audio_processing_impl.cc
@@ -1849,7 +1849,8 @@
           echo_control_factory_->Create(proc_sample_rate_hz());
     } else {
       private_submodules_->echo_controller = absl::make_unique<EchoCanceller3>(
-          EchoCanceller3Config(), proc_sample_rate_hz());
+          EchoCanceller3Config(), proc_sample_rate_hz(),
+          /*num_render_channels=*/1, /*num_capture_channels=*/1);
     }
 
     capture_nonlocked_.echo_controller_enabled = true;