Set public visibility for R8 desugar binary

We are migrating Android functionality out of Bazel into rules_android.
Currently rules_android depends on
@bazel_tools//tools/android:desugar_java8 for a sh_binary that calls the
R8 desugar binary. Going forward, we'll maintain the sh_binary directly
in rules_android, and therefore will require public visibility on
[...]/r8:desugar. Eventually the source code for the Bazel R8 desugar
wrapper will also move into rules_android.

Part of bazelbuild/rules_android#122.

src/tools/android/java/com/google/devtools/build/android/r8/BUILD.tools

@@ -46,7 +46,7 @@ java_binary(
         "-Dcom.android.tools.r8.createSingletonsForStatelessLambdas",
     ],
     main_class = "com.google.devtools.build.android.r8.Desugar",
-    visibility = ["//tools/android:__subpackages__"],
+    visibility = ["//visibility:public"],
     runtime_deps = [
         ":r8",
     ],