Fix swipe-to-home not enter pip from split properly

Unlick 3-button home-key to enter pip from split flow, if using gesture
navigation, the flow is like:
  swipe to home -> handle as a TYPE_RECENTS_DURING_SPLIT ->
  finishTransition checks and triggers auto-pip -> create TRANSIT_PIP ->
  reparent task to TDA -> collect to new transition -> file TRANSIT_PIP
Since the task was collected after reparent, shell-side unable to
identify if it was in split when receiving TRANSIT_PIP.
This checks if a TRANSIT_PIP implies empty one side of the split, so we
can handle enter-pip-from-split properly.

Fix: 287002454
Test: 1. Return to home from a split pair, the activity on the
         split can enter PiP and clear the split pair in overview.
      2. Put a split pair in background, launch another
         fullscreen activity and request entering PiP, the log
         should not show:
         "Got a PiP-enter request while Split-Screen is active".
Change-Id: Ifc129e85a69e7c05363204e72fe4cc6ba61b9b47