Upgrade rust/crates/protobuf-codegen to 2.20.0

Test: make
Change-Id: I2aaabad83af5a304ad847943933dbe5a116ec575
diff --git a/Android.bp b/Android.bp
index 412db99..c6dcac6 100644
--- a/Android.bp
+++ b/Android.bp
@@ -2,6 +2,7 @@
 
 rust_library_host {
     name: "libprotobuf_codegen",
+    // has rustc warnings
     crate_name: "protobuf_codegen",
     srcs: ["src/lib.rs"],
     edition: "2015",
@@ -12,6 +13,7 @@
 
 rust_test_host {
     name: "protobuf-codegen_host_test_src_lib",
+    // has rustc warnings
     crate_name: "protobuf_codegen",
     srcs: ["src/lib.rs"],
     test_suites: ["general-tests"],
@@ -24,6 +26,7 @@
 
 rust_binary_host {
     name: "protoc-gen-rust",
+    // has rustc warnings
     crate_name: "protoc_gen_rust",
     srcs: ["src/bin/protoc-gen-rust.rs"],
     edition: "2015",
@@ -34,4 +37,4 @@
 }
 
 // dependent_library ["feature_list"]
-//   protobuf-2.18.1
+//   protobuf-2.20.0