Round animation progress for computing size for background

Round background crop animation progress when it's less then
epsilon to match fullscreen size and eliminate a line of couple
pixels high bleeding through from an app underneath. This
happens when animating letterboxed app from recents and can
be visible at the end of animation below nav bar.

Bug: 63156733
Bug: 62893418
Test: go/wm-smoke
Test: Trigger animations to/from recents with different rotations.
Change-Id: Ia4e93c81eac93a886007d53da3a685a69d4cb4de