Destroy prefetched layers when the context is destroyed

Also fix a few places preventing the tests from running
in SkiaVulkan, although that is still not enabled in general yet

Fixes: 283315634
Test: CanvasContext_buildLayerDoesntLeak unit test
Change-Id: I4d8a50db447e47afd8e8a267df332a173c88e888
diff --git a/libs/hwui/tests/unit/RenderNodeTests.cpp b/libs/hwui/tests/unit/RenderNodeTests.cpp
index 80796f4..8273524 100644
--- a/libs/hwui/tests/unit/RenderNodeTests.cpp
+++ b/libs/hwui/tests/unit/RenderNodeTests.cpp
@@ -231,8 +231,7 @@
 }
 
 TEST(RenderNode, releasedCallback) {
-    int functor = WebViewFunctor_create(
-            nullptr, TestUtils::createMockFunctor(RenderMode::OpenGL_ES), RenderMode::OpenGL_ES);
+    int functor = TestUtils::createMockFunctor();
 
     auto node = TestUtils::createNode(0, 0, 200, 400, [&](RenderProperties& props, Canvas& canvas) {
         canvas.drawWebViewFunctor(functor);