[RenderEngine] Organize RenderEngine directory.

This patch:
1. adds proper namespace renderengine to all RenderEngine code,
   and namespace gl to all GLES related code
2. creates gl/ directory for GLES backend code
3. Reorder include header files, remove unused header files.

BUG: 112585051
Test: Build, flash, run display validation
Change-Id: I81f0b8831213607cde08562958f7c38ddaf4c9e6
diff --git a/services/surfaceflinger/LayerBE.h b/services/surfaceflinger/LayerBE.h
index 531cdcd..f5150a7 100644
--- a/services/surfaceflinger/LayerBE.h
+++ b/services/surfaceflinger/LayerBE.h
@@ -91,13 +91,13 @@
     explicit LayerBE(const LayerBE& layer);
 
     void onLayerDisplayed(const sp<Fence>& releaseFence);
-    void clear(RE::RenderEngine& renderEngine);
-    Mesh& getMesh() { return mMesh; }
+    void clear(renderengine::RenderEngine& renderEngine);
+    renderengine::Mesh& getMesh() { return mMesh; }
 
 private:
     Layer*const mLayer;
     // The mesh used to draw the layer in GLES composition mode
-    Mesh mMesh;
+    renderengine::Mesh mMesh;
 
     // HWC items, accessed from the main thread
     struct HWCInfo {