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/Android.mk b/services/surfaceflinger/Android.mk
index fcc9241..1cd9215 100644
--- a/services/surfaceflinger/Android.mk
+++ b/services/surfaceflinger/Android.mk
@@ -15,6 +15,7 @@
     Layer.cpp \
     LayerDim.cpp \
     LayerRejecter.cpp \
+    LayerVector.cpp \
     MessageQueue.cpp \
     MonitoredProducer.cpp \
     SurfaceFlingerConsumer.cpp \