Support fade-in divider bar with quick switch gesture

Fix: 229613465
Test: quickswitch back to split pair, divider bar will fade-in as
      expected.
Change-Id: I2746ef63ecf9a9cc5563c543cc83583955e0c938
diff --git a/quickstep/src/com/android/quickstep/TaskAnimationManager.java b/quickstep/src/com/android/quickstep/TaskAnimationManager.java
index 2565674..54f457d 100644
--- a/quickstep/src/com/android/quickstep/TaskAnimationManager.java
+++ b/quickstep/src/com/android/quickstep/TaskAnimationManager.java
@@ -192,6 +192,10 @@
                                 RemoteAnimationTargetCompat.wrap(nonAppTargets) /* nonApps */);
                         return;
                     }
+                } else if (nonAppTargets != null && nonAppTargets.length > 0) {
+                    TaskViewUtils.createSplitAuxiliarySurfacesAnimator(
+                            RemoteAnimationTargetCompat.wrap(nonAppTargets) /* nonApps */,
+                            true /*shown*/, dividerAnimator -> dividerAnimator.start());
                 }
                 if (mController != null) {
                     if (mLastAppearedTaskTarget == null