SF: Separate RenderEngine into interface and impl

This allows the RenderEngine to be substituted by a GMock for tests.

RE::RenderEngine is now a pure virtual interface class.
RE::impl::RenderEngine is the normal/base implementation.

Similarly, RE::Image and RE::Surface are pure virtual interfaces.
RE::impl::Image and RE::impl::Surface are the normal implementations.

Test: Builds
Bug: None
Change-Id: Ib5e658df4bb4efc1a9c0ae95feaf0c1e052cdc94
diff --git a/services/surfaceflinger/Layer.h b/services/surfaceflinger/Layer.h
index c63399e..3671a2b 100644
--- a/services/surfaceflinger/Layer.h
+++ b/services/surfaceflinger/Layer.h
@@ -90,7 +90,7 @@
         hwc_color_t color;
     } hwc;
     struct {
-        RenderEngine* renderEngine;
+        RE::RenderEngine* renderEngine;
         Mesh* mesh;
     } renderEngine;
 };