app launcher icon changed to default icon on Oreo

For API 26+ default Android application template defines another icon resource folder

mipmap-anydpi-v26

That folder (usually) contains two xml files ic_launcher and ic_launcher_round that match icon resources declared in manifest for API 26+

Content of those files looks like following:

<?xml version="1.0" encoding="utf-8"?>
<adaptive-icon xmlns:android="http://schemas.android.com/apk/res/android">
    <background android:drawable="@drawable/ic_launcher_background"/>
    <foreground android:drawable="@drawable/ic_launcher_foreground"/>
</adaptive-icon>

In order to have your icon you also need to change all drawables (in all density buckets) listed there - namely ic_launcher_background and ic_launcher_foreground

Or you can just delete that folder, in which case android will fallback to using your png icons, but they will not appear as-is and will be commonly drawn on top of white background.

You can read more at: Adaptive icons

Best solution is delete mipmap-anydpi-v26 folder then app will take default icon. In android studio Project mode go to this package

res/mipmap-anydpi-v26

delete it and rebuild and Run Project.