Make software codecs as VNDK

Software codecs and their dependencies are marked as VNDK (or just
vendor_available:true for static/header libs).

Bug: 37343418
Test: build the software codecs with BOARD_VNDK_VERSION=current
Change-Id: I9e628d0646fd8a28f8e741cde5ef0c30b79c21ae
diff --git a/Android.bp b/Android.bp
index b4a8922..c9310e1 100644
--- a/Android.bp
+++ b/Android.bp
@@ -1,5 +1,9 @@
 cc_library_shared {
     name: "libopus",
+    vendor_available: true,
+    vndk: {
+        enabled: true,
+    },
 
     export_include_dirs: ["include"],