For those, who want to do it automatically, I have wrote a little two-lines bash script which does next two things:
Renames extracted classes.jar to be like the aar but with a new extension
find . -name '*.aar' -exec sh -c 'unzip -d `dirname {}` {} classes.jar' \;
find . -name '*.aar' -exec sh -c 'mv `dirname {}`/classes.jar `echo {} | sed s/aar/jar/g`' \;
That's it!