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: If64eff62b9e93af95089e5010c30eb586673b92e
diff --git a/libc/Android.bp b/libc/Android.bp
index b90908e..f814089 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -1769,6 +1769,13 @@
     system_shared_libs: [],
     stl: "none",
     sdk_version: "current",
+
+    apex_available: [
+        "//apex_available:platform",
+        "com.android.runtime",
+        "com.android.art.release", // from libdexfile_external
+        "com.android.art.debug", // from libdexfile_external
+    ],
 }
 
 // libc_headers for libasync_safe and libpropertyinfoparser