Preserve the leash reference in PipTaskOrganizer

Make sure to not lose the reference to updated leash
upon user manually entering PiP with another PiP window
present.

Also remove the leash of the previous PiP window.

This will also avoid calling onExitPipFinished() twice,
since it is already being called through onTaskVanished()
in the case of overriding an existing PiP.

Bug: 285231679
Test: manually reproduce the steps in the bug
Change-Id: I581759a5ae121779e7151007be133f0f5c6f9ee5