You would have to get the material library jar file, because it doesn't exist in the current sources.
You will find its Aar file here, you can try to extract the classes.jar file, and use it with your extension ( for the app Inventor Sources see here, for the extension template see this topic, if you are using Appybuilder, so it's not possible ):
https://mvnrepository.com/artifact/com.google.android.material/material/1.3.0
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