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/sysprop/Android.bp b/sysprop/Android.bp
index a00a5e4..22cba3b 100644
--- a/sysprop/Android.bp
+++ b/sysprop/Android.bp
@@ -21,4 +21,6 @@
         "sysprop_test.go",
     ],
     pluginFor: ["soong_build"],
+    // Used by plugins
+    visibility: ["//visibility:public"],
 }