Zomg, way late but I spent 2 hours figuring this out and want to share!
I used Android Studio to create the icon, starting from a new blank project. I was able to figure out how to get to the icon generator from the description here, though I feel like I got lucky finding it, so if you need it, you can pay the $40 to view the video to see where that interface is. Then, I copied the files generated from the /app/src/main/res folder in that project, to my /res/icons/android directory.
Then, I added this to config.xml (I have other things inside this block too, but this is the relevant stuff:
<platform name="android">
<!-- Depend on v21 of appcompat-v7 support library -->
<framework src="com.android.support:appcompat-v7:21+" />
<edit-config file="app/src/main/AndroidManifest.xml" mode="merge" target="/manifest/application">
<application android:icon="@mipmap/ic_launcher" android:roundIcon="@mipmap/ic_launcher_round" />
</edit-config>
<resource-file src="res/icons/android/drawable/ic_launcher_background.xml" target="app/src/main/res/drawable/ic_launcher_background.xml" />
<resource-file src="res/icons/android/drawable-v24/ic_launcher_foreground.xml" target="app/src/main/res/drawable-v24/ic_launcher_foreground.xml" />
<resource-file src="res/icons/android/mipmap-anydpi-v26/ic_launcher_round.xml" target="app/src/main/res/mipmap-anydpi-v26/ic_launcher_round.xml" />
<resource-file src="res/icons/android/mipmap-anydpi-v26/ic_launcher.xml" target="app/src/main/res/mipmap-anydpi-v26/ic_launcher.xml" />
<resource-file src="res/icons/android/mipmap-hdpi/ic_launcher_foreground.png" target="app/src/main/res/mipmap-hdpi/ic_launcher_foreground.png" />
<resource-file src="res/icons/android/mipmap-hdpi/ic_launcher_round.png" target="app/src/main/res/mipmap-hdpi/ic_launcher_round.png" />
<resource-file src="res/icons/android/mipmap-hdpi/ic_launcher.png" target="app/src/main/res/mipmap-hdpi/ic_launcher.png" />
<resource-file src="res/icons/android/mipmap-mdpi/ic_launcher_foreground.png" target="app/src/main/res/mipmap-mdpi/ic_launcher_foreground.png" />
<resource-file src="res/icons/android/mipmap-mdpi/ic_launcher_round.png" target="app/src/main/res/mipmap-mdpi/ic_launcher_round.png" />
<resource-file src="res/icons/android/mipmap-mdpi/ic_launcher.png" target="app/src/main/res/mipmap-mdpi/ic_launcher.png" />
<resource-file src="res/icons/android/mipmap-xhdpi/ic_launcher_foreground.png" target="app/src/main/res/mipmap-xhdpi/ic_launcher_foreground.png" />
<resource-file src="res/icons/android/mipmap-xhdpi/ic_launcher_round.png" target="app/src/main/res/mipmap-xhdpi/ic_launcher_round.png" />
<resource-file src="res/icons/android/mipmap-xhdpi/ic_launcher.png" target="app/src/main/res/mipmap-xhdpi/ic_launcher.png" />
<resource-file src="res/icons/android/mipmap-xxhdpi/ic_launcher_foreground.png" target="app/src/main/res/mipmap-xxhdpi/ic_launcher_foreground.png" />
<resource-file src="res/icons/android/mipmap-xxhdpi/ic_launcher_round.png" target="app/src/main/res/mipmap-xxhdpi/ic_launcher_round.png" />
<resource-file src="res/icons/android/mipmap-xxhdpi/ic_launcher.png" target="app/src/main/res/mipmap-xxhdpi/ic_launcher.png" />
<resource-file src="res/icons/android/mipmap-xxxhdpi/ic_launcher_foreground.png" target="app/src/main/res/mipmap-xxxhdpi/ic_launcher_foreground.png" />
<resource-file src="res/icons/android/mipmap-xxxhdpi/ic_launcher_round.png" target="app/src/main/res/mipmap-xxxhdpi/ic_launcher_round.png" />
<resource-file src="res/icons/android/mipmap-xxxhdpi/ic_launcher.png" target="app/src/main/res/mipmap-xxxhdpi/ic_launcher.png" />
</platform>
I'm not fully sure if the appcompat line is necessary so you can try without it if you like. I did also need to remove the android platform & re-add each time I made a change, to get it to notice the change. At one point I also deleted the folder .idea/caches to clear cache. All that, and now it works!! I think it's a better solution than the accepted answer here, because you aren't mucking with the platforms folder & thus won't need to hand-copy things every time, once and done. Hope this saves someone some time!