Added script for generate key.properties file.

tool/generate_android_key_properties.dart

@@ -0,0 +1,14 @@
+import 'dart:io';
+
+const outputPath = 'android/key.properties';
+
+Future<void> main(List<String> args) async {
+  final output = args.fold('', (String acc, String arg) => acc + arg + '\n');
+  final outputFile = File(outputPath);
+
+  if (outputFile.existsSync()) {
+    await outputFile.delete();
+  }
+
+  await outputFile.writeAsString(output);
+}