Fixes OpenActivityEmbeddingSecondaryToSplitTest flakiness

It should be optional for the app window to be invisible
before turning to visible.

Bug: 329929112
Test: OpenActivityEmbeddingSecondaryToSplitTest

Change-Id: Ib82f24afc014884d32de6694fa0d9b9279e00936

tests/FlickerTests/ActivityEmbedding/src/com/android/server/wm/flicker/activityembedding/open/OpenActivityEmbeddingSecondaryToSplitTest.kt

@@ -90,7 +90,10 @@ class OpenActivityEmbeddingSecondaryToSplitTest(flicker: LegacyFlickerTest) :
         flicker.assertWm {
             notContains(ActivityEmbeddingAppHelper.SECONDARY_ACTIVITY_COMPONENT)
                 .then()
-                .isAppWindowInvisible(ActivityEmbeddingAppHelper.SECONDARY_ACTIVITY_COMPONENT)
+                .isAppWindowInvisible(
+                    ActivityEmbeddingAppHelper.SECONDARY_ACTIVITY_COMPONENT,
+                    isOptional = true
+                )
                 .then()
                 .isAppWindowVisible(ActivityEmbeddingAppHelper.SECONDARY_ACTIVITY_COMPONENT)
         }