Prevent SurfaceFlinger from using layer token 31.

Bug: 2964479
Change-Id: I81e948924bff35b8b300d409f2c09f3779bcdeec
diff --git a/services/surfaceflinger/SurfaceFlinger.cpp b/services/surfaceflinger/SurfaceFlinger.cpp
index 4876946..4b3235d 100644
--- a/services/surfaceflinger/SurfaceFlinger.cpp
+++ b/services/surfaceflinger/SurfaceFlinger.cpp
@@ -2427,7 +2427,7 @@
             }
             break;
         }
-        if (++name > 31)
+        if (++name >= SharedBufferStack::NUM_LAYERS_MAX)
             name = NO_MEMORY;
     } while(name >= 0);