Most of you may use some free icon providers like icons8, flat icons, etc...
Those icons need to be credited somewhere in your app. And that was the work done by someone.
If you even don't have any knowledge in icon designing there is a tool available for generating your app icons for FREE.
For creating awesome looking icons for your android apps can be challenging without the right tools and experience.
Roman Nurik made a tool that makes this work really simple. He is a part of Google's Design Team and made many other tools for developers.




As represented in the above image, you can make varieties of app icons with density variations that are used in Android Studio projects.
This tool has Material Icons in the clip art section or if you want you can upload your own logo in the Image section.
You can also make Icons only with the letters (Text).
You have other editing options like padding, trimming, effects like shadows, etc...
After you complete just download the .zip file and extract and copy it to your Android Studio project res folders.

Visit Android Asset Generator Website 

It's an Opensource project and if you want to how this tool works you can have a look at its code which is committed to Github AAG