Add post_update.sh

* replace CARGO_PKG_VERSION with version string from METADATA

Test: tools/external_updater/updater.sh update --refresh --keep_date rust/crates/protobuf-codegen
Change-Id: Ic2d5728a327d001b626d33907403c2ced61925ce
diff --git a/src/lib.rs b/src/lib.rs
index 4fc3ee9..cff1da2 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -186,9 +186,7 @@
     {
         let mut w = CodeWriter::new(&mut v);
 
-        // ANDROID: hard code version number here because Android.bp
-        // rust modules cannot pass it though env variable yet.
-        w.write_generated_by("rust-protobuf", "2.17.0");
+        w.write_generated_by("rust-protobuf", "2.17.0"); // ANDROID ported version
         w.write_line(&format!("//! Generated file from `{}`", file.get_name()));
         if customize.inside_protobuf != Some(true) {
             w.write_line("");