When a project is reset, the client deletes all the files from its dir
that BOINC knows about.
However, there may be other files, created by its apps.
On Android, where space is likely to be scarce,
delete these when the project is reset.
Maybe we should do this on all projects.
But we'd need to not to delete user-created files
like app_config.xml, and anonymous-platform stuff
like app_info.xml and the app files.