Changed mNumLayers to atomic

mNumLayers is accessed from multiple threads, binder thread when a layer
is created and main thread when a layer is destroyed. The variable
should be atomic to ensure it's thread safe.

Test: Builds and runs
Change-Id: I72962d148dfdeec8e1877b86f93cc53c8c7883a9
diff --git a/services/surfaceflinger/SurfaceFlinger.h b/services/surfaceflinger/SurfaceFlinger.h
index 3974a8c..03f620a 100644
--- a/services/surfaceflinger/SurfaceFlinger.h
+++ b/services/surfaceflinger/SurfaceFlinger.h
@@ -1072,7 +1072,7 @@
     // Static screen stats
     bool mHasPoweredOff = false;
 
-    size_t mNumLayers = 0;
+    std::atomic<size_t> mNumLayers = 0;
 
     // Verify that transaction is being called by an approved process:
     // either AID_GRAPHICS or AID_SYSTEM.