Looks fine for now. Going forward I would refactor the API and make Mount an interface and move isMounted to Mount. For now we can go with this change so that property index work is not blocked
Scratch that missed seeing the other patch. As discussed one thing we need to manage is how this isMounted is implemented for default mount. As for that it needs to consult other mounts to determine if anyone claims that path, if not then it part of default mount. So we would need to do away with the DEFAULT Mount as a constant and instead have a method MountInfoProvider#getDefaultMount
MountInfoProvider defines the API for getting mount info by other Oak parts
Initial implementation done with 648f207929