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/LayerState.cpp b/libs/gui/LayerState.cpp
index 3949186..16bf324 100644
--- a/libs/gui/LayerState.cpp
+++ b/libs/gui/LayerState.cpp
@@ -28,7 +28,7 @@
     output.writeUint32(what);
     output.writeFloat(x);
     output.writeFloat(y);
-    output.writeUint32(z);
+    output.writeInt32(z);
     output.writeUint32(w);
     output.writeUint32(h);
     output.writeUint32(layerStack);
@@ -54,7 +54,7 @@
     what = input.readUint32();
     x = input.readFloat();
     y = input.readFloat();
-    z = input.readUint32();
+    z = input.readInt32();
     w = input.readUint32();
     h = input.readUint32();
     layerStack = input.readUint32();