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/ISurfaceComposer.h b/include/gui/ISurfaceComposer.h
index 824e5c4..ff04e2f 100644
--- a/include/gui/ISurfaceComposer.h
+++ b/include/gui/ISurfaceComposer.h
@@ -157,7 +157,7 @@
     virtual status_t captureScreen(const sp<IBinder>& display,
             const sp<IGraphicBufferProducer>& producer,
             Rect sourceCrop, uint32_t reqWidth, uint32_t reqHeight,
-            uint32_t minLayerZ, uint32_t maxLayerZ,
+            int32_t minLayerZ, int32_t maxLayerZ,
             bool useIdentityTransform,
             Rotation rotation = eRotateNone) = 0;