SurfaceFlinger: Add ChildColorLayerTest with zero alpha

The alpha value of its parent layer is not considered in isVisible as it
is in setGeometry. It results in that HWC gets a ColorLayer with 0.0f in
alpha, which doesn't need to be displayed.

Bug: b/110632776
Test: Run sffakehwc_test w/ and w/o the fix
Change-Id: I91a4be00ae303917e6aa5094d67b01604581b776