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}

modules/audio_processing/audio_processing_impl_unittest.cc

@@ -60,6 +60,12 @@ class MockEchoControlFactory : public EchoControlFactory {
     return mock;
   }
 
+  std::unique_ptr<EchoControl> Create(int sample_rate_hz,
+                                      size_t num_render_channels,
+                                      size_t num_capture_channels) override {
+    return Create(sample_rate_hz);
+  }
+
  private:
   std::unique_ptr<MockEchoControl> next_mock_;
 };