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/phony/Android.bp b/phony/Android.bp
index db5efc9..2e250c6 100644
--- a/phony/Android.bp
+++ b/phony/Android.bp
@@ -13,4 +13,5 @@
         "phony.go",
     ],
     pluginFor: ["soong_build"],
+    visibility: ["//visibility:public"],
 }