Bluetooth: Audio: Add receive_state_updated for scan delegator

Add the receive_state_updated callback for the scan delegator.
This callback will be called whenever a receive state is updated
by any means, giving the upper layers the option to always know
the latest changes.

This commit also refactors the name used for the internal
receive state (which contains additional information), for
the sake of readability.

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