Preemptively reset split reparenting flag when entering split

- Currently, this flag is not reset until we start the animation or when
  the stage visibilities officially change, but following a drop to
  initiate split and Shell actually receiving the transition,
  the app may launch a trampoline activity (ie. esp during create)
  which will trigger the task to be reparented back out of the
  split root (preventing split from succeeding).  Instead we preemptively
  reset this state when we know that we are about to enter split, and
  rely on the existing paths to update the flag when the split is next
  fully shown or hidden.

Bug: 292454704
Test: Drag app that launches a trampoline into split
Change-Id: I17c8b7e92dfbf18a1bd1a9ca64b7ee7f3097f233