Store scopeToCancel.

Without this the scope that gets cancelled can be the new one after re-attaching. This results in no flows working - hence blank thumbnails.

Fix: 369590189
Flag: com.android.launcher3.enable_refactor_task_thumbnail
Test: Manual - visually verified
Change-Id: Ie930d890095c30c4be8e87c795a06383ac06ea9e

quickstep/src/com/android/quickstep/task/thumbnail/TaskThumbnailView.kt

@@ -127,8 +127,9 @@ class TaskThumbnailView : FrameLayout, ViewPool.Reusable {
 
     override fun onDetachedFromWindow() {
         super.onDetachedFromWindow()
+        val scopeToCancel = viewAttachedScope
         recentsCoroutineScope.launch(dispatcherProvider.background) {
-            viewAttachedScope.cancel("TaskThumbnailView detaching from window")
+            scopeToCancel.cancel("TaskThumbnailView detaching from window")
         }
     }