fix failing lint in MainActivity.kt.tmpl (#159441)

`what android studio lint is failing`
![Capture d’écran 2024-11-25
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Author: AbdeMohlbi

packages/flutter_tools/templates/app_shared/android-kotlin.tmpl/app/src/main/kotlin/androidIdentifier/MainActivity.kt.tmpl

@@ -2,4 +2,4 @@ package {{androidIdentifier}}
 
 import io.flutter.embedding.android.FlutterActivity
 
-class MainActivity: FlutterActivity()
+class MainActivity : FlutterActivity()