Use TYPE_ENTER_PIP_FROM_SPLIT only if the task is on split

Otherwise even if the split pair is in background, any standalone
entering PiP will always use TYPE_ENTER_PIP_FROM_SPLIT. That
may either disturb split state or notify some unexpected transition
handlers to run additional animation.

Bug: 286363453
Test: 1. Return to home from a split pair, the activity on the
         split can enter PiP successfully.
      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: I780d0bd22ba5e3992ad859e6f770033c1b4d6487