Use vndk.private: true for VNDK-private libs

Instead of using `vendor_available: false`, use `vndk.private: true`
for VNDK-private libraries.

Bug: 175768895
Test: build
Change-Id: I7fedc67769c7e031182b2b3a040fe085caefe2ba
diff --git a/libs/gui/Android.bp b/libs/gui/Android.bp
index 4a4510e..18ade37 100644
--- a/libs/gui/Android.bp
+++ b/libs/gui/Android.bp
@@ -32,9 +32,10 @@
 
 cc_library_shared {
     name: "libgui",
-    vendor_available: false,
+    vendor_available: true,
     vndk: {
         enabled: true,
+        private: true,
     },
     double_loadable: true,