Use protobuf-codegen default and add mod_stem.rs

* Upgrade to new protobuf-codegen; use its standard
  default output without local change.
* Allow a sourceProvider to have multiple output files.
  For a stem.proto file, output stem.rs and mod_stem.rs.
* New protobuf-codegen option gen_mod_rs always generates
  output file named "mod.rs". To generate multiple .proto
  files into the same output directory, we need to rename
  mod.rs to mod_<stem>.rs.
* Instead of using the gen_mod_rs option and renaming mod.rs
  to mod_<stem>.rs, we generate the same mod_<stem>.rs
  directly with a simple printf command.

Bug: 170256643
Test: atest -c --host --include-subdirs external/crosvm
Change-Id: Ia09e41029099a6de4d35c96dbabd9ba5514c9019
diff --git a/rust/protobuf.go b/rust/protobuf.go
index 897300f..ebb1c3c 100644
--- a/rust/protobuf.go
+++ b/rust/protobuf.go
@@ -61,15 +61,22 @@
 	}
 
 	outDir := android.PathForModuleOut(ctx)
-	depFile := android.PathForModuleOut(ctx, proto.BaseSourceProvider.getStem(ctx)+".d")
-	outputs := android.WritablePaths{android.PathForModuleOut(ctx, proto.BaseSourceProvider.getStem(ctx)+".rs")}
+	stem := proto.BaseSourceProvider.getStem(ctx)
+	// rust protobuf-codegen output <stem>.rs
+	stemFile := android.PathForModuleOut(ctx, stem+".rs")
+	// add mod_<stem>.rs to import <stem>.rs
+	modFile := android.PathForModuleOut(ctx, "mod_"+stem+".rs")
+	// mod_<stem>.rs is the main/first output file to be included/compiled
+	outputs := android.WritablePaths{modFile, stemFile}
+	depFile := android.PathForModuleOut(ctx, "mod_"+stem+".d")
 
 	rule := android.NewRuleBuilder()
 	android.ProtoRule(ctx, rule, protoFile.Path(), protoFlags, protoFlags.Deps, outDir, depFile, outputs)
+	rule.Command().Text("printf '// @generated\\npub mod %s;\\n' '" + stem + "' >").Output(modFile)
 	rule.Build(pctx, ctx, "protoc_"+protoFile.Path().Rel(), "protoc "+protoFile.Path().Rel())
 
-	proto.BaseSourceProvider.OutputFile = outputs[0]
-	return outputs[0]
+	proto.BaseSourceProvider.OutputFiles = android.Paths{modFile, stemFile}
+	return modFile
 }
 
 func (proto *protobufDecorator) SourceProviderProps() []interface{} {