[perf] FlutterExternalIdeActionGroup: make action update thread explicit (#8236)

Making the action update thread explicit saves the platform from the
work of figuring it out for itself (required caching of potentially
unneeded data). From a related inspection:

> Reports actions, action groups and other ActionUpdateThreadAware
classes that do not explicitly state their mode.
> When an action or an action group defines its own update() method,
IntelliJ Platform tries to mimic the old synchronous way of calling
update() and getChildren() methods in the UI thread and supply it with
all the data in AnActionEvent.dataContext(). To do that, it caches all
the possible data on a background thread beforehand, even if it is not
needed.
> Provide one of the two modes ActionUpdateThread.EDT or
ActionUpdateThread.BGT by overriding the getActionUpdateThread() method.



![image](https://github.com/user-attachments/assets/444c53e0-5c80-489f-8b3e-82419cb18b1b)


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Author: pq

flutter-idea/src/io/flutter/actions/FlutterExternalIdeActionGroup.java

@@ -5,10 +5,7 @@
  */
 package io.flutter.actions;
 
-import com.intellij.openapi.actionSystem.AnActionEvent;
-import com.intellij.openapi.actionSystem.CommonDataKeys;
-import com.intellij.openapi.actionSystem.DefaultActionGroup;
-import com.intellij.openapi.actionSystem.Presentation;
+import com.intellij.openapi.actionSystem.*;
 import com.intellij.openapi.project.Project;
 import com.intellij.openapi.vfs.VirtualFile;
 import io.flutter.FlutterUtils;
@@ -36,6 +33,11 @@ private static boolean isExternalIdeFile(AnActionEvent e) {
       FlutterUtils.isXcodeProjectFileName(file.getName()) || OpenInAndroidStudioAction.isProjectFileName(file.getName());
   }
 
+  @Override
+  public @NotNull ActionUpdateThread getActionUpdateThread() {
+    return ActionUpdateThread.EDT;
+  }
+
   public static boolean isAndroidDirectory(@NotNull VirtualFile file) {
     return file.isDirectory() && (file.getName().equals("android") || file.getName().equals(".android"));
   }