Add default_visibility for soong modules

So that we don't get surprise usages.

Bug: 348717861
Test: m nothing
Change-Id: I47319a727092c1bd936cca88f5713ee977a48b48
diff --git a/java/metalava/Android.bp b/java/metalava/Android.bp
index ccbd191..6bf1832 100644
--- a/java/metalava/Android.bp
+++ b/java/metalava/Android.bp
@@ -15,4 +15,5 @@
 filegroup {
     name: "metalava-config-files",
     srcs: ["*-config.xml"],
+    visibility: ["//visibility:public"],
 }