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/source_provider.go b/rust/source_provider.go
index 03adf9e..436518c 100644
--- a/rust/source_provider.go
+++ b/rust/source_provider.go
@@ -30,7 +30,7 @@
 type BaseSourceProvider struct {
 	Properties SourceProviderProperties
 
-	OutputFile       android.Path
+	OutputFiles      android.Paths
 	subAndroidMkOnce map[SubAndroidMkProvider]bool
 	subName          string
 }
@@ -43,11 +43,11 @@
 	SourceProviderProps() []interface{}
 	SourceProviderDeps(ctx DepsContext, deps Deps) Deps
 	setSubName(subName string)
-	setOutputFile(outputFile android.Path)
+	setOutputFiles(outputFiles android.Paths)
 }
 
 func (sp *BaseSourceProvider) Srcs() android.Paths {
-	return android.Paths{sp.OutputFile}
+	return sp.OutputFiles
 }
 
 func (sp *BaseSourceProvider) GenerateSource(ctx ModuleContext, deps PathDeps) android.Path {
@@ -97,6 +97,6 @@
 	sp.subName = subName
 }
 
-func (sp *BaseSourceProvider) setOutputFile(outputFile android.Path) {
-	sp.OutputFile = outputFile
+func (sp *BaseSourceProvider) setOutputFiles(outputFiles android.Paths) {
+	sp.OutputFiles = outputFiles
 }