Change PiP stash scoring to distance moved

When there are multiple possible stashing positions for a PiP,
before we chose the one that displaces the PiP by the least area
(dx * height + dy * width).
With eg. very tall expanded PiPs, this can lead a large dy to be
preferred over a small dx because of the big height multiplier,
causing the PiP to move a 'greater distance'.
This change makes the stashing position with the least `dx + dy`
preferred.

Bug: 226579596
Test: manual
Change-Id: I29f74a2bec670c3a3219c6d50bf1102a41d42b6b