Mark libmemtrack as VNDK in Android.bp

As a VNDK module, Android.bp must have 'vndk' tag as well as
'vendor_available: true'.

The 'vndk' tag for VNDK module is formated as below:
vndk: {
    enabled: true,
},

VNDK modules will be installed both in system/lib(64) as normal and
in system/lib(64)/vndk as a vendor variant.

Bug: 63866913
Test: build and boot with BOARD_VNDK_VERSION=current
Merged-In: I4b9e560ca6d4751889a7b14f205e678b68c20008
Change-Id: I4b9e560ca6d4751889a7b14f205e678b68c20008
(cherry picked from commit 7aeb5bb86c)

libmemtrack/Android.bp

@@ -2,6 +2,10 @@
 
 cc_library_shared {
     name: "libmemtrack",
+    vendor_available: true,
+    vndk: {
+        enabled: true,
+    },
     srcs: ["memtrack.cpp"],
     export_include_dirs: ["include"],
     local_include_dirs: ["include"],