add a layerStack attribute to Layers.

this attribute can be set through a regular transaction using
SurfaceComposerClient (just like any other attribute, eg: position or size)

Change-Id: I701a47c677ea6442ca713728a93335328cd2b172
diff --git a/include/gui/SurfaceComposerClient.h b/include/gui/SurfaceComposerClient.h
index 9f3ef35..aef4223 100644
--- a/include/gui/SurfaceComposerClient.h
+++ b/include/gui/SurfaceComposerClient.h
@@ -119,6 +119,7 @@
     status_t    setPosition(SurfaceID id, float x, float y);
     status_t    setSize(SurfaceID id, uint32_t w, uint32_t h);
     status_t    setCrop(SurfaceID id, const Rect& crop);
+    status_t    setLayerStack(SurfaceID id, uint32_t layerStack);
     status_t    destroySurface(SurfaceID sid);
 
 private: