Make solid color cache attempts use alpha

Without alpha, there was no shader being compiled at all.

Test: Confirm no jank camera open, close, open with perfetto.

Bug: b/189912149

Change-Id: Ib17572bed7393b4b8709095d3bd6480a1141ad72
diff --git a/libs/renderengine/skia/Cache.cpp b/libs/renderengine/skia/Cache.cpp
index 61c297c..8a41395 100644
--- a/libs/renderengine/skia/Cache.cpp
+++ b/libs/renderengine/skia/Cache.cpp
@@ -149,6 +149,7 @@
                     PixelSource{
                             .solidColor = half3(0.1f, 0.2f, 0.3f),
                     },
+            .alpha = 0.5,
     };
 
     auto layers = std::vector<const LayerSettings*>{&layer};