SF: Break out NativeWindowSurface

I should perhaps have done this originally.
Separating it out like this  is better for the upcoming factory change.

Test: atest libsurfaceflinger_unittest
Bug: None

Change-Id: Ia0d87c55cab9b315886eba6320b5e345632e4231
diff --git a/services/surfaceflinger/Android.bp b/services/surfaceflinger/Android.bp
index 4f12d3e..339f83b 100644
--- a/services/surfaceflinger/Android.bp
+++ b/services/surfaceflinger/Android.bp
@@ -138,6 +138,7 @@
         "LayerStats.cpp",
         "LayerVector.cpp",
         "MonitoredProducer.cpp",
+        "NativeWindowSurface.cpp",
         "RenderArea.cpp",
         "Scheduler/DispSync.cpp",
         "Scheduler/DispSyncSource.cpp",