Bluetooth: Audio: Refactor scan delegator set sync state

The bt_bap_scan_delegator_set_sync_state has been refactored
to bt_bap_scan_delegator_set_bis_sync_state, as it will,
going forward, only be used to set the BIS sync states,
and not the PA sync state. The PA sync state will, in a future
commit, but autonousmly handled by the scan delegator
based on the PA sync callbacks.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>