add appIcon manifest placeholder

Author: JulianKast

android/hello_sdl_android/src/main/AndroidManifest.xml

@@ -28,7 +28,7 @@
 
     <application
         android:allowBackup="false"
-        android:icon="@mipmap/ic_launcher"
+        android:icon="${appIcon}"
         android:label="@string/app_name"
         android:theme="@style/AppTheme"
         tools:ignore="DeepLinks">