Unfortunately, it’s not that easy. I guess you would have to exclude the dependency ical4j from ical4android and include a new submodule (= the downloaded local ical4j) instead.
Could you, please, implement two-way syncronization with the local ISC file?
It would be usefull for contacts backup and peer-to-peer contacts syncronization.
DAVdroid is a CalDAV/CardDAV sync adapter. Synchronization with local sources is not planned at the moment. Please do backups and contact sharing on the server side.
Requested feature could make possible serverless decentralized peer-to-peer syncronization using external tool like Syncthing, BTSync, etc.