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