...
<platform name="android">
  ...
  <resource-file src="resources/android/notification/drawable-hdpi/notification_icon.png" target="app/src/main/res/drawable-hdpi/notification_icon.png" />
  <resource-file src="resources/android/notification/drawable-mdpi/notification_icon.png" target="app/src/main/res/drawable-mdpi/notification_icon.png" />
  <resource-file src="resources/android/notification/drawable-xhdpi/notification_icon.png" target="app/src/main/res/drawable-xhdpi/notification_icon.png" />
  <resource-file src="resources/android/notification/drawable-xxhdpi/notification_icon.png" target="app/src/main/res/drawable-xxhdpi/notification_icon.png" />
  <resource-file src="resources/android/notification/drawable-xxxhdpi/notification_icon.png" target="app/src/main/res/drawable-xxxhdpi/notification_icon.png" />
  <config-file parent="./application" target="AndroidManifest.xml">
    <meta-data android:name="com.google.firebase.messaging.default_notification_icon" android:resource="@drawable/notification_icon" />
  </config-file>