Disable rounded corners when in multi window

We should not draw rounded corners when animating windows into split
view. The divider is a single horizontal line, and we don't support
multiple radii per window.

Fixes: 129540518
Test: visual
Change-Id: Ic31395c77f376053a136a9c55382b74b4157ee34
Merged-In: Ic31395c77f376053a136a9c55382b74b4157ee34