Reduce no-op IPC when swiping up

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

This avoids a latency from the 2-way IPC before calling
RecentsAnimationListener#onRecentsAnimationStart.

Bug: 275508603
Test: Swipe-up
Change-Id: If1aa847bd92b2a16812a5181b4476d9a35619a76