Launcher icon is changed but in recent apps still default icon i seen
The app icon in the app launcher should be updated, but the shortcuts won't be updated until the next refresh of the home-screen. There isn't much you can do that unfortunately, but the home-screen does refresh itself once in a while, so the new icon will eventually replace the old icon automatically.
Reboot your phone and your icon will update in recent apps and notification.
From Android Nougat, default launcher icon should be roundIcon. See this Reference
Maybe you should change your AndroidManifest for your application tag
android:icon="@mipmap/ic_launcher" android:roundIcon="@mipmap/ic_launcher_round"