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/androidmk.go b/rust/androidmk.go
index 5a33f77..29e4bd7 100644
--- a/rust/androidmk.go
+++ b/rust/androidmk.go
@@ -156,7 +156,7 @@
 }
 
 func (sourceProvider *BaseSourceProvider) AndroidMk(ctx AndroidMkContext, ret *android.AndroidMkData) {
-	outFile := sourceProvider.OutputFile
+	outFile := sourceProvider.OutputFiles[0]
 	ret.Class = "ETC"
 	ret.OutputFile = android.OptionalPathForPath(outFile)
 	ret.SubName += sourceProvider.subName