Correct AEC3 multichannel functionality activation

This CL corrects the AEC3 multichannel activation
to also work for the case when a factory is used
for the activation.

Bug: webrtc:10913
Change-Id: Ic2807d8bcef759261fde14447cff30633ba248dc
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/158794
Commit-Queue: Per Åhgren <peah@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29676}

modules/audio_processing/audio_processing_impl_unittest.cc

@@ -54,6 +54,12 @@ class MockEchoControlFactory : public EchoControlFactory {
   // Returns a pointer to the next MockEchoControl that this factory creates.
   MockEchoControl* GetNext() const { return next_mock_.get(); }
   std::unique_ptr<EchoControl> Create(int sample_rate_hz) override {
+    RTC_NOTREACHED();
+    return nullptr;
+  }
+  std::unique_ptr<EchoControl> Create(int sample_rate_hz,
+                                      int num_render_channels,
+                                      int num_capture_channels) override {
     std::unique_ptr<EchoControl> mock = std::move(next_mock_);
     next_mock_ = std::make_unique<MockEchoControl>();
     return mock;