Add default_visibility for soong modules

So that we don't get surprise usages.

Bug: 348717861
Test: m nothing
Change-Id: I47319a727092c1bd936cca88f5713ee977a48b48

rust/Android.bp

@@ -61,4 +61,5 @@ bootstrap_go_package {
         "test_test.go",
     ],
     pluginFor: ["soong_build"],
+    visibility: ["//visibility:public"],
 }

rust/config/Android.bp

@@ -24,4 +24,8 @@ bootstrap_go_package {
         "x86_64_device.go",
         "arm64_linux_host.go",
     ],
+    visibility: [
+        "//build/soong:__subpackages__",
+        "//prebuilts/rust/soong",
+    ],
 }