VirtualDisplayTaskEmbedder: Post background color change callback

Otherwise we will end up calling SurfaceView#setResizeBackgroundColor
from a Binder thread and potentially cause a crash. This mirrors the
behavior of TaskOrganizerTaskEmbedder.

Bug: 157605189
Test: Existing tests pass
Change-Id: Iafe9e4c3eb4d32a52a6a1479aa9a2b295cd4ad6f

core/java/android/window/VirtualDisplayTaskEmbedder.java

@@ -365,8 +365,8 @@ public class VirtualDisplayTaskEmbedder extends TaskEmbedder {
             // Found the topmost stack on target display. Now check if the topmost task's
             // description changed.
             if (taskInfo.taskId == stackInfo.taskIds[stackInfo.taskIds.length - 1]) {
-                mHost.onTaskBackgroundColorChanged(VirtualDisplayTaskEmbedder.this,
-                        taskInfo.taskDescription.getBackgroundColor());
+                mHost.post(()-> mHost.onTaskBackgroundColorChanged(VirtualDisplayTaskEmbedder.this,
+                        taskInfo.taskDescription.getBackgroundColor()));
             }
         }