Set apex_available property

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

Bug: 150999716
Test: m

Change-Id: Ib121c4b300a7ca540e095068dac377b060e4dca4
diff --git a/transport/safe_union/1.0/Android.bp b/transport/safe_union/1.0/Android.bp
index 2760863..d0aa348 100644
--- a/transport/safe_union/1.0/Android.bp
+++ b/transport/safe_union/1.0/Android.bp
@@ -11,4 +11,8 @@
         "types.hal",
     ],
     gen_java: true,
+    apex_available: [
+        "//apex_available:anyapex",
+        "//apex_available:platform",
+    ],
 }