SurfaceFlinger: Fix GL composition of ColorLayers

Fixes an issue where the alpha for ColorLayers during
GL composition was taken from the drawing state instead
of walking the hierarchy.

This resulted in ColorLayers being drawn at the wrong
opacity during transitions whenever we dropped out of
HWC.

Bug: 74220420
Test: Developer Settings > Disable HW overlays, then launch Chrome from Home, repeatedly, verify no black frame is visible.
Change-Id: Iec94812485fa27af08cb72f875e7be3b946822a7