Merge emulated cutout into either rounded corner overlay

This so that we can save a layer, which avoids dropping us into
GL composition during animations.

This assumes that the cutout is always at the top or bottom edge
in the device's natural orientation.

Note that the two overlays for the top and bottom rounded corners
are still separate.

Bug: 72492508
Test: enable emulated cutout, verify it still shows up
Change-Id: I895084828e0502005bfa31e37d23dd3a6f01a2ca