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

This is a reland of f3a197e55323aee974a932c52dd19fa88e5d4e38

Original change's description:
> 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}

Bug: webrtc:10913
Change-Id: Ifc4b13bd994cfd22dca8f8755fa5700617cc379d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/151124
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Commit-Queue: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29034}
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;