Match window rounded corners with system rounded corners.

Use the corner radius defined in ScreenDecorationsUtils as the rounded
corner radius to keep caption rounded corner in line with other sources
of rounded corners.

Bug: 280825814
Test: Manual; open an app that uses the system rounded corners;
confirm it lines up with caption corner.

Change-Id: Id15a7173cc72a59fef73571cac281f5f08dba48f