SurfaceFlinger: Use traversal functions to iterate LayerList.

In preparation for the Layer hierarchy. There we will need
to use such a style to traverse the tree of layers.

Test: Just a refactoring. SurfaceFlinger still works.
Change-Id: I84dcd82e713f1bdbe911658793ce11460267a956
diff --git a/services/surfaceflinger/Layer.cpp b/services/surfaceflinger/Layer.cpp
index 5f32119..5b61c25 100644
--- a/services/surfaceflinger/Layer.cpp
+++ b/services/surfaceflinger/Layer.cpp
@@ -903,7 +903,7 @@
 
         // figure out if there is something below us
         Region under;
-        const SurfaceFlinger::LayerVector& drawingLayers(
+        const LayerVector& drawingLayers(
                 mFlinger->mDrawingState.layersSortedByZ);
         const size_t count = drawingLayers.size();
         for (size_t i=0 ; i<count ; ++i) {