[ads][CodeHealth] Separate callback type ownership by build layer
Removes callback type aliases from `core/public/ads_callback.h` that
are no longer used by the `Ads` public interface. Separates callback
type ownership by build layer: `ads_service_callback.h` (always-built)
owns the callback types for `AdsService`, and `ads_callback.h`
(ads-only, with `static_assert(BUILDFLAG(ENABLE_BRAVE_ADS))`) owns the
callback types for `Ads`. This prevents the always-built `AdsService`
from including ads-only headers. No behavioral changes.
Terry Mancey
8399272cbd