Fix regression in Assistant MicView.

Copy and paste error caused Assistant MicView to never register to
observe AssistantInteractionModel events in recent refactoring.

Bug: b:157579976
Change-Id: I992387d69ccfafccd8842070563269b65f974aa2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2218673Reviewed-by: Jeroen Dhollander <jeroendh@google.com>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#772479}

ash/assistant/ui/dialog_plate/mic_view.cc

@@ -33,7 +33,7 @@ MicView::MicView(AssistantButtonListener* listener, AssistantButtonId button_id)
   InitLayout();
 
   assistant_controller_observer_.Add(AssistantController::Get());
-  AssistantInteractionController::Get()->GetModel()->RemoveObserver(this);
+  AssistantInteractionController::Get()->GetModel()->AddObserver(this);
 }
 
 MicView::~MicView() {