[pigeon] Separates message call code generation into separate methods in the KotlinGenerator (flutter#5891)

Separates message call code generation into separate methods in the KotlinGenerator for flutter#134777. The ProxyApi generator uses similar code to the HostApi and FlutterApi, so this makes the code reusable.

From suggestion: flutter/packages#5544 (comment)

Author: bparrishMines

packages/pigeon/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 16.0.3
+
+* [kotlin] Separates message call code generation into separate methods.
+
 ## 16.0.2
 
 * [dart] Separates message call code generation into separate methods.

packages/pigeon/lib/generator_tools.dart

@@ -13,7 +13,7 @@ import 'ast.dart';
 /// The current version of pigeon.
 ///
 /// This must match the version in pubspec.yaml.
-const String pigeonVersion = '16.0.2';
+const String pigeonVersion = '16.0.3';
 
 /// Read all the content from [stdin] to a String.
 String readStdin() {