Support layer occlusion with rounded corners

Currently, when a layer is covered by another with rounded corners, both are marked only as covered and never as occluded. With the change in flickerlib to support occlusion in this state (SF fix still pending) the test needs to be updated to reflect this scenario

Also, disable flaky flicker test because of recent regression

Test: atest WMShellFlickerTests
Bug: 265007895
Change-Id: I98b97ef3e2d6cbe084811ca7e658fa6d590a8060