Split AppListController::SetVisible into Show and Dismiss.

This CL is in preparation for passing a reason for showing the app list.
This split is necessary to pass a reason only when showing and not
when dismissing.

BUG=391348

Review URL: https://codereview.chromium.org/485453003

Cr-Commit-Position: refs/heads/master@{#292080}
diff --git a/ash/shell.cc b/ash/shell.cc
index 1f9ef2c4..6aca4b217 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -298,13 +298,13 @@
     window = GetTargetRootWindow();
   if (!app_list_controller_)
     app_list_controller_.reset(new AppListController);
-  app_list_controller_->SetVisible(true, window);
+  app_list_controller_->Show(window);
 }
 
 void Shell::DismissAppList() {
   if (!app_list_controller_)
     return;
-  app_list_controller_->SetVisible(false, GetTargetRootWindow());
+  app_list_controller_->Dismiss();
 }
 
 void Shell::ToggleAppList(aura::Window* window) {