Fix signedness issue in LayerProtoParser

Makes things appear out of order in the dump.

Test: Manual use of dumpsys.
Change-Id: I48b29281cddf2f66a88e21e162858205fbace25d
diff --git a/services/surfaceflinger/layerproto/LayerProtoParser.cpp b/services/surfaceflinger/layerproto/LayerProtoParser.cpp
index e6f2ce7..bf37e1e 100644
--- a/services/surfaceflinger/layerproto/LayerProtoParser.cpp
+++ b/services/surfaceflinger/layerproto/LayerProtoParser.cpp
@@ -29,9 +29,11 @@
     uint32_t rs = rhs->layerStack;
     if (ls != rs) return ls < rs;
 
-    uint32_t lz = lhs->z;
-    uint32_t rz = rhs->z;
-    if (lz != rz) return lz < rz;
+    int32_t lz = lhs->z;
+    int32_t rz = rhs->z;
+    if (lz != rz) {
+        return (lz > rz) ? 1 : -1;
+    }
 
     return lhs->id < rhs->id;
 }