surfaceflinger: fix z type in LayerVector::do_compare

The issue was exposed by commit 80de6f3 (Fix surfaceflinger on
integer sanitized builds).

Test: SurfaceFlinger_test
Change-Id: Ie77b2f48f2108030a308360c07cf0260f33c48ca
diff --git a/services/surfaceflinger/LayerVector.cpp b/services/surfaceflinger/LayerVector.cpp
index d0f8fbe..47156c1 100644
--- a/services/surfaceflinger/LayerVector.cpp
+++ b/services/surfaceflinger/LayerVector.cpp
@@ -37,8 +37,8 @@
     if (ls != rs)
         return (ls > rs) ? 1 : -1;
 
-    uint32_t lz = l->getCurrentState().z;
-    uint32_t rz = r->getCurrentState().z;
+    int32_t lz = l->getCurrentState().z;
+    int32_t rz = r->getCurrentState().z;
     if (lz != rz)
         return (lz > rz) ? 1 : -1;