I downloaded collection of icons from Android Developers center Each icon in this collection is ordered in forlders according to its resolution: drawable-hdpi, drawable-mdpi
Actually if you downloaded the icons pack from the android web site, you will see that you have one folder per resolution named drawable-mdpi etc. Copy all folders into the res (not the drawable) folder in Android Studio. This will automatically make all the different resolution of the icon available.
what u need to do is icons downloaded from material design, open that folder there are lots of icons categories specified, open any of it choose any icon and go to this folder -> drawable-anydpi-v21. this folder contains xml files copy any xml file and paste it to this location -> C:\Users\Username\AndroidStudioProjects\ur project name\app\src\main\res\drawable. That's it !! now you can use the icon in ur project.