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/aidl_library/Android.bp b/aidl_library/Android.bp
index ec21504..07472a4 100644
--- a/aidl_library/Android.bp
+++ b/aidl_library/Android.bp
@@ -29,4 +29,5 @@
         "aidl_library_test.go",
     ],
     pluginFor: ["soong_build"],
+    visibility: ["//visibility:public"],
 }