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: I7f9e4326e1428886ce32f625ffcee98ea2c82a9a
diff --git a/media/libmedia/Android.bp b/media/libmedia/Android.bp
index fb4fe4b..9086e2f 100644
--- a/media/libmedia/Android.bp
+++ b/media/libmedia/Android.bp
@@ -121,6 +121,9 @@
 cc_library_shared {
     name: "libmedia_omx",
     vendor_available: true,
+    vndk: {
+        enabled: true,
+    },
 
     defaults: ["libmedia_omx_defaults"],
 }