PiP: Divide up the magnetic percent by 2.

At max size, doing a brute percentage calculation using width / minWidth
ends up with a magnetic distance that's too big. Divide by 2 again to
make it more natural.

Bug: 190657668
Test: Pinch to max size, drag down - doesn't immediate snap to it
anymore

Change-Id: Ib5de742b753239a151686c1a85b27a6fe553a577