Implement runtime switch to select default renderer mode

Add a system property debug.hwui.default_renderer, which allows
to set rendering mode to OpenGL (default), Skia OpenGL or Vulkan.

Change-Id: I8bca5bacc5108f77437e340ac61f2d8db8cc4c39
diff --git a/libs/hwui/renderthread/RenderProxy.cpp b/libs/hwui/renderthread/RenderProxy.cpp
index 10a17f8..c689544 100644
--- a/libs/hwui/renderthread/RenderProxy.cpp
+++ b/libs/hwui/renderthread/RenderProxy.cpp
@@ -55,7 +55,7 @@
 
 CREATE_BRIDGE4(createContext, RenderThread* thread, bool translucent,
         RenderNode* rootRenderNode, IContextFactory* contextFactory) {
-    return new CanvasContext(*args->thread, args->translucent,
+    return CanvasContext::create(*args->thread, args->translucent,
             args->rootRenderNode, args->contextFactory);
 }