Allow separate top and bottom rounded.xml layouts

ScreenDecorations expects that the overlays for the top and bottom
(containing the rounded corners and potentially display cutout) will
have the exact same drawable on all 4 corners. This change allows for
two separate layouts divided into top and bottom.

Test: manual
Bug: 156017682
Change-Id: Icbfaf66b19a6d154dd67ce2d9bc013a29778c329