1. Go to: Android Asset Studio

• upload your custom white icon.
• Name: ic_stat_your_icon_name
• Click Download to get the zip file.

2. Extract it and copy the drawable-* folders into your Flutter Android app at:

android/app/src/main/res/drawable-hdpi/ic_stat_your_icon_name.png
android/app/src/main/res/drawable-mdpi/ic_stat_your_icon_name.png

Notification icons are white-only with transparent background