Rework public async API
Right now, (nearly all) the async _start() calls require using a callback, and calling the appropriate _finish() function. Ideally, those would be called automatically when the operation has finished, and the _finish() call would only be needed to capture the result, not to set the fingerprint driver's state machine back in the right state.
This would make the async functions similar to GIO's "GTask" handling.
Benefits:
- Generic callbacks for finished operations (but new callbacks for enrollment progress)
- Less confusion for front-end developers
- Clearer match between sync/async, and when to call
_finish()