Actually apply version script

Remove absent symbols - they were never exported. Explicitly add
private/ symbols to LIBNATIVEWINDOW_PLATFORM version.

Bug: http://b/69603741
Test: make && boot taimen
Change-Id: I6ff7c170a5ad5f0bcee5d65e27aef04310286a21
diff --git a/libs/nativewindow/Android.bp b/libs/nativewindow/Android.bp
index 64eb110..0eadbb0 100644
--- a/libs/nativewindow/Android.bp
+++ b/libs/nativewindow/Android.bp
@@ -44,6 +44,8 @@
         "-std=c++1z"
     ],
 
+    version_script: "libnativewindow.map.txt",
+
     srcs: [
         "AHardwareBuffer.cpp",
         "ANativeWindow.cpp",