For Android 6, the default synchronization interval for OpenTasks is set to
DEFAULT_SYNC_INTERVAL (currently 4 hours) if OpenTasks is installed, and
“manually” if OpenTasks is not installed.
So, if you install OpenTasks after DAVdroid, but before creating the DAVdroid account, DAVdroid will set the default sync interval automatically. However, if you add an account and then install OpenTasks, you’ll have to change the sync interval of the existing account manually.
For Android <6, behavior stays the same: OpenTasks sync is always disabled when OpenTasks is not accessible (and it won’t be accessible unless OpenTasks was installed before DAVdroid was installed).