Mark the module as VNDK or VNDK-SP in Android.bp

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

For a VNDK module, the 'vndk' tag has 'enabled: true'.
It will be installed system/lib(64)/vndk as a vendor variant.

For a VNDK-SP module, the 'vndk' tag has
'support_system_process: true' as well as 'enabled: true'.
It will be installed system/lib(64)/vndk-sp as a vendor variant.

Bug: 63866913
Test: build and boot with BOARD_VNDK_VERSION=current
Change-Id: I82e0d42be49ab578d8797d596e9915a60900bb47
diff --git a/Android.bp b/Android.bp
index 4f159c8..ca2702e 100644
--- a/Android.bp
+++ b/Android.bp
@@ -16,6 +16,9 @@
     name: "libevent",
     host_supported: true,
     vendor_available: true,
+    vndk: {
+        enabled: true,
+    },
     unique_host_soname: true,
 
     arch: {