Disallow enter PiP when starting a split task

Disallow enter PiP while Launching another task
into one of split stage tasks.

This should also help Core keep
ActivityTaskSupervisor#mUserLeaving as false,
which helps avoid onUserLeaveHint() on client-side
and any legacy enter-PiP requests.

Bug: 336477473
Test: repro the steps in the bug (latest comments)

Change-Id: Idceebc9742f45d17ebe5398df2c151085d33c775