Break Layer class into Gl and Vulkan subclasses

Test: manual testing
Change-Id: Ibd2beed39de3ac6da7448e96496253cfe427dfbb
diff --git a/libs/hwui/OpenGLReadback.h b/libs/hwui/OpenGLReadback.h
index f4ebabc..c9222cf 100644
--- a/libs/hwui/OpenGLReadback.h
+++ b/libs/hwui/OpenGLReadback.h
@@ -22,7 +22,7 @@
 namespace uirenderer {
 
 class Matrix4;
-class Layer;
+class GlLayer;
 
 class OpenGLReadback : public Readback {
 public:
@@ -49,7 +49,7 @@
     /**
      * Copies the layer's contents into the provided bitmap.
      */
-    static bool copyLayerInto(renderthread::RenderThread& renderThread, Layer& layer,
+    static bool copyLayerInto(renderthread::RenderThread& renderThread, GlLayer& layer,
             SkBitmap* bitmap);
 
 protected: