Can I restore deleted files (undo a `git clean -fdx`)?

No. Those files are gone.

(Just checked on Linux: git clean calls unlink(), and does not backup up anything beforehand.)

git clean -fdxn

Will do a dry run, and show you what files would be deleted if you ran

git clean -fdx

(of course this is only helpful if you know this ahead of time, but for next time)

IntelliJ/Android Studio allows restoring files from local history.