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/ISurfaceComposer.h b/include/gui/ISurfaceComposer.h
index de1d65d..0d4a76b 100644
--- a/include/gui/ISurfaceComposer.h
+++ b/include/gui/ISurfaceComposer.h
@@ -67,6 +67,7 @@
         eMatrixChanged              = 0x00000010,
         eTransparentRegionChanged   = 0x00000020,
         eVisibilityChanged          = 0x00000040,
+        eLayerStackChanged          = 0x00000080,
         eCropChanged                = 0x00000100,
     };