API review fixes

- Remove the notion of channel from an application perspective
- Remove the use of DnsSd as part of API and add a protocol type to allow
future extensions
- Use the listener object as a handler for stop and unregister operations. Also,
we now allow multiple active discovery operations.
- Remove init/deinit operations in favour of setting up construction

Bug:6434844
Change-Id: Id15521d571aac9c4053d81184ad66dfc54f636de