Make kotlinx_atomicfu available to apexes

kotlinx_atomicfu is used when kotlinx_coroutines is built from
source, make it available to apexes.

Test: m checkbuild
Change-Id: I12262a578276aa15d503fe50535662b643857447
diff --git a/Android.bp b/Android.bp
index 94614fa..23d101a 100644
--- a/Android.bp
+++ b/Android.bp
@@ -32,4 +32,8 @@
         "-Xmulti-platform",
     ],
     visibility: ["//external/kotlinx.coroutines"],
+    apex_available: [
+        "//apex_available:platform",
+        "//apex_available:anyapex",
+    ],
 }