Starting with Android 5 (Lollipop), Android requires small icons used within notifications to contain only white or transparent "pixels" (source). This results in square shaped icons to be used by default with a mobile app, since most of these are square shaped, with no transparency.

In order to overcome this, we must first create a new icon, possibly based off of the previous one, but with a transparent background. Then, we can use this online tool to create all the required sizes and turn all colored pixels white, packaging them into a zip archive.

Finally, we take the "dummy-android-plugin.zip" archive on this great post from Carlos Alfaro and made some adjustments:

1. We unzipped the contents of the 'res' folder on the obtained zip archive onto the root of "dummy-android-plugin.zip";
2. We adjusted the "resource-file" tags, on the plugin.xml file, from "dummy-android-plugin.zip", to:
   1. Declare "icon.png" files on both "target" and "src" attributes;
   2. Delete the "-v4" portions from the "target" attributes.

This resulted in the attached zip archive, which can be used in a similar manner from the original post, by adding it as a resource with the corresponding extensibility configurations. Then all you need is to reference it on the home module of your mobile app, like you would any other plugin!