Floss: Some light code clean ups

* Removes unnecessary #[allow(dead_code)]. The new macro behaves
  differently that this warning supporession is not needed anymore.

* `unregister_scanner_callback` parameter should be named `callback_id`
  instead of `scanner_id`. This becomes more important to disambiguate
  since there will be `unregister_scanner` that takes `scanner_id`.

* `callbacks` should be named `scanner_callbacks` to disambiguate with
  other types of Gatt callbacks.

* Add missing D-Bus projection RegisterScanner and UnregisterScanner in
  btclient.

Bug: 217273154
Tag: #floss
Test: Manual - Build Floss on Linux

Change-Id: I5094a3f806698fb94c0e51cf78262a26b24eeb27