Temporarily add support for conditional modules

Some bootclasspath libraries are currently conditionally included
in the SDK. Add support for that in api.go.

Bug: 169103987
Test: mmm frameworks/base/api
Change-Id: I7704a884675eb58c429a59f99a663cf09807ecec
diff --git a/api/Android.bp b/api/Android.bp
index 83009f5..7aa68f8 100644
--- a/api/Android.bp
+++ b/api/Android.bp
@@ -126,6 +126,9 @@
         "framework-wifi",
         "i18n.module.public.api",
     ],
+    conditional_bootclasspath: [
+        "framework-supplementalapi",
+    ],
     system_server_classpath: [
         "service-media-s",
         "service-permission",