Set apex_available property

The marked library(ies) were available to the APEXes via the
hand-written whitelist in build/soong/apex/apex.go. Trying to remove the
whitelist by adding apex_available property to the Android.bp of the
libraries.

Bug: 150999716
Test: m
Change-Id: I7abed722266cbe04e862508a6ee7f745dd807e8f
diff --git a/Android.bp b/Android.bp
index cf2b253..ec224af 100644
--- a/Android.bp
+++ b/Android.bp
@@ -29,6 +29,11 @@
     srcs: [
         ":liblua-sources"
     ],
+
+    apex_available: [
+        "//apex_available:platform",
+        "com.android.extservices",
+    ],
 }
 
 filegroup {