Notifications have changed in Android 5 and 6 from Android 4. Icons now must be white and there are two icons (small and large).
The default for the small icon is app's icon. And default large icon is small icon. However, if app's icon is colored, notification shows a default bell icon.
This can be changed if you supply a custom png. According to OneSignal docs, in Cordova or Ionic project, custom large icon should reside in <project-root>/platforms/android/res/drawable-xxxhdpi/ic_onesignal_large_icon_default.png.
But I cannot find where to put it in a XDK project. I tried to put in www root folder, and in res folder, but didn't work. No docs were found either.
See this thread > https://software.intel.com/en-us/forums/intel-xdk/topic/680309 < you need to create a custom plugin that will copy the icon files into the project, similar to what the example in that thread had to do in order to replace a sound file.