Allow SkiaVkRenderEngine to not be compiled

A device will use RenderEngine with either SkiaVk or SkiaGL, but never
both at the same time.  Given that the choice is controlled by a
build-time value in the device's "device.mk" file, to save space, we
should allow the RenderEngine back-ends to be optionally built.  For
now, we'll only do this for SkiaVkRenderEngine.

Test: Compile
Bug: 259323498
Change-Id: Id547feaee7e8aeb4e46759d3d7c9c5dbe18f5211
diff --git a/libs/renderengine/RenderEngine.cpp b/libs/renderengine/RenderEngine.cpp
index d08c221..341c011 100644
--- a/libs/renderengine/RenderEngine.cpp
+++ b/libs/renderengine/RenderEngine.cpp
@@ -39,8 +39,12 @@
             ALOGD("RenderEngine with SkiaGL Backend");
             return renderengine::skia::SkiaGLRenderEngine::create(args);
         case RenderEngineType::SKIA_VK:
+#ifdef RE_SKIAVK
             ALOGD("RenderEngine with SkiaVK Backend");
             return renderengine::skia::SkiaVkRenderEngine::create(args);
+#else
+            LOG_ALWAYS_FATAL("Requested VK backend, but RE_SKIAVK is not defined!");
+#endif
         case RenderEngineType::SKIA_GL_THREADED: {
             ALOGD("Threaded RenderEngine with SkiaGL Backend");
             return renderengine::threaded::RenderEngineThreaded::create(
@@ -50,12 +54,16 @@
                     args.renderEngineType);
         }
         case RenderEngineType::SKIA_VK_THREADED:
+#ifdef RE_SKIAVK
             ALOGD("Threaded RenderEngine with SkiaVK Backend");
             return renderengine::threaded::RenderEngineThreaded::create(
                     [args]() {
                         return android::renderengine::skia::SkiaVkRenderEngine::create(args);
                     },
                     args.renderEngineType);
+#else
+            LOG_ALWAYS_FATAL("Requested VK backend, but RE_SKIAVK is not defined!");
+#endif
         case RenderEngineType::GLES:
         default:
             ALOGD("RenderEngine with GLES Backend");