What is the location of the keystore file in Android Studio?

There's a good chance you'll find it in "C:\Users\.gradle\gradle.properties". From Android Studio, you can find that file in "Gradle Scripts->gradle.properties(Global Properties)

If you used the apk after clicking "Build Apk", its a debug build apk from the output folder. Or if you used "Generate Signed Apk" you must have used a key as there is an option to create one.

I found mine C:\Users\username\keystores\myappkey.jks

Try to check the menu: Build -> Generate Signed APK

The path to the keystore would be displayed on the first line of the new window.

But if you don't remember the passwords then I have some bad news for you :-(