SurfaceFlinger and libgui: Switch Z-order to signed type.

In preparation for SurfaceFlinger child layers. In that model
children's Z order will be relative to their parent. We need
negative Z values to represent children stacking below their
parent (e.g. SurfaceView). Java side already uses signed types
strangely enough.

Test: Basically a refactoring. SurfaceFlinger still works.
Change-Id: Ifcece69f6f9d917cbf5238a59f8e5de1e8ba6a25
diff --git a/include/gui/SurfaceControl.h b/include/gui/SurfaceControl.h
index 456bde4..62217ad 100644
--- a/include/gui/SurfaceControl.h
+++ b/include/gui/SurfaceControl.h
@@ -61,7 +61,7 @@
     void        disconnect();
 
     status_t    setLayerStack(uint32_t layerStack);
-    status_t    setLayer(uint32_t layer);
+    status_t    setLayer(int32_t layer);
     status_t    setLayerInfo(uint32_t type, uint32_t appid);
     status_t    setPosition(float x, float y);
     status_t    setSize(uint32_t w, uint32_t h);