Rounded corner child clipping tests

Fixed original rounded corners test - I assumed that a Rect was expressed in
Rect(x, y, w, h) instead of Rect(l, t, r, b) - and introduced new test that
verifies if child layers are clipped correctly.

Bug: 118461793
Bug: 111514493
Test: /data/nativetest64/SurfaceFlinger_test/SurfaceFlinger_test
Change-Id: I2dfde220e0d56c2aa0d48ae7b4006a97e04039d2