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/libs/gui/SurfaceControl.cpp b/libs/gui/SurfaceControl.cpp
index b47e434..c82f6a7 100644
--- a/libs/gui/SurfaceControl.cpp
+++ b/libs/gui/SurfaceControl.cpp
@@ -102,7 +102,7 @@
     if (err < 0) return err;
     return mClient->setLayerStack(mHandle, layerStack);
 }
-status_t SurfaceControl::setLayer(uint32_t layer) {
+status_t SurfaceControl::setLayer(int32_t layer) {
     status_t err = validate();
     if (err < 0) return err;
     return mClient->setLayer(mHandle, layer);