Respect LayerSettings::alpha in Skia-RE for buffer layers

Test: Manual. Open notification. Check whether the alpha of
      the shade is correct.
      Bug: 173418499

Change-Id: I66c184ec1328647311fa1e342b4f0cd3d1bb7aed
diff --git a/libs/renderengine/skia/SkiaGLRenderEngine.cpp b/libs/renderengine/skia/SkiaGLRenderEngine.cpp
index 0c89cd4..579260b 100644
--- a/libs/renderengine/skia/SkiaGLRenderEngine.cpp
+++ b/libs/renderengine/skia/SkiaGLRenderEngine.cpp
@@ -677,6 +677,8 @@
             } else {
                 paint.setShader(shader);
             }
+            // Make sure to take into the account the alpha set on the layer.
+            paint.setAlphaf(layer->alpha);
         } else {
             ATRACE_NAME("DrawColor");
             const auto color = layer->source.solidColor;