Mark libpng as VNDK

This lib is used by some OEMs. So mark it as VNDK so that we can make
the lib available to them, while keeping the lib in the system
partition.

Bug: 63866913
Test: BOARD_VNDK_VERSION=current m -j libpng libpng.vendor
Change-Id: I6bf73db6118747206b6817a55a7f327e3a68f05b
diff --git a/Android.bp b/Android.bp
index 65bb9ba..a5c78ee 100644
--- a/Android.bp
+++ b/Android.bp
@@ -79,6 +79,10 @@
 
 cc_library {
     name: "libpng",
+    vendor_available: true,
+    vndk: {
+        enabled: true,
+    },
     host_supported: true,
     defaults: ["libpng-defaults"],
     target: {