[R8] Orphan proguard_mappings_output_file

[vadymsavchenko-lyft]

With the recent change to the android_binary rule's default config (commit)

proguard_mappings_output_file is no longer written to the baseline bazel-bin and now lands in the transitioned ST-<hash> output root, so it is no longer under the directory bazel info bazel-bin reports. More generally the mapping is an output of no target: it can't be requested by label, never enters a runfiles tree, and isn't fetched under --remote_download_toplevel.

The solution is to declare proguard_mappings_output_file as implicit_outputs of the ProviderInfo, copying the D8 approach. This doesn't change where the file is written; it makes it a default output, so consumers in the top-level configuration can collect it.

diff --git a/rules/android_binary/r8.bzl b/rules/android_binary/r8.bzl
index 4c4510114..c921d6d98 100644
--- a/rules/android_binary/r8.bzl
+++ b/rules/android_binary/r8.bzl
@@ -194,6 +194,7 @@ def process_r8(ctx, validation_ctx, jvm_ctx, packaged_resources_ctx, build_info_
         value = struct(
             final_classes_dex_zip = final_classes_dex_zip,
             dex_info = android_dex_info,
+            implicit_outputs = [proguard_mappings_output_file],
             providers = [
                 android_dex_info,
                 AndroidPreDexJarInfo(pre_dex_jar = deploy_jar),