I've spent the whole afternoon on this problem - done everything that it says in the Push Plugin documentation and more - and it appears that putting an extra tag in your Android manifest fixes the problem:
<meta-data android:name="com.google.firebase.messaging.default_notification_icon"
android:resource="@drawable/icontrans" />
... where icontrans is the name of my icon.
For reference, I also put the icons into each of the res/drawable folders using the following in my config.xml:
<resource-file src="res/icon/icon-18-trans.png" target="res/drawable-ldpi/icontrans.png" />
<resource-file src="res/icon/icon-24-trans.png" target="res/drawable-mdpi/icontrans.png" />
<resource-file src="res/icon/icon-36-trans.png" target="res/drawable-hdpi/icontrans.png" />
<resource-file src="res/icon/icon-48-trans.png" target="res/drawable-xhdpi/icontrans.png" />
<resource-file src="res/icon/icon-72-trans.png" target="res/drawable-xxhdpi/icontrans.png" />
<resource-file src="res/icon/icon-96-trans.png" target="res/drawable-xxxhdpi/icontrans.png" />
Not sure if it's something to do with the later platforms (I'm targeting SDK 26) or the fact I'm using Firebase to send the notifications.