Only manipulate leashes for leaf-tasks when adapting to legacy

We only wrap leaf-tasks for legacy animations; however, I forgot
to update some places so they were trying to use non-leaf leashes.

Bug: 229758341
Test: atest ActivityVisibilityTests#testTurnScreenOnAttrNoLockScreen_SplitScreen
Change-Id: Iee5c08c8dad268b798b372311a80ddede1a4beaa