I was able to solve it myself...

First I had to include the path to my directory inside the asterisk-java project directory: sudo java -cp asterisk-java.jar:guava-33.3.1-jre.jar:/usr/src/asterisk-java/target/buildPackage org.asteriskjava.fastagi.DefaultAgiServer

Then, in fastagi-mapping.properties I had to declare the function class in the following way:

callin.agi=org.asteriskjava.examples.fastagi.java.ExampleCallIn

Be aware that if you're using the -cp flag with java or javac, the path that follows overrides the user classpath that is stored in the $CLASSPATH variable.