Reduce redundant IPC when switching app by navigation bar

In SplitScreenController#onStartingSplitLegacy, it also ignores:
if (ENABLE_SHELL_TRANSITIONS) return null;

This avoids a latency from the 2-way IPC in
RecentsAnimationListener#onTasksAppeared.

Bug: 275508603
Test: Capture trace when switching app by quickstep.

Change-Id: Ia11cbca0d38cdc2bfd676dfa3d59a5c8931a2d6e
diff --git a/quickstep/src/com/android/quickstep/TaskAnimationManager.java b/quickstep/src/com/android/quickstep/TaskAnimationManager.java
index 8e03a91..69db91b 100644
--- a/quickstep/src/com/android/quickstep/TaskAnimationManager.java
+++ b/quickstep/src/com/android/quickstep/TaskAnimationManager.java
@@ -206,8 +206,9 @@
                     }
                 }
 
-                RemoteAnimationTarget[] nonAppTargets = SystemUiProxy.INSTANCE.get(mCtx)
-                        .onStartingSplitLegacy(appearedTaskTargets);
+                RemoteAnimationTarget[] nonAppTargets = ENABLE_SHELL_TRANSITIONS
+                        ? null : SystemUiProxy.INSTANCE.get(mCtx).onStartingSplitLegacy(
+                                appearedTaskTargets);
                 if (nonAppTargets == null) {
                     nonAppTargets = new RemoteAnimationTarget[0];
                 }