add missing public interface for iOS client app id

The methods are already in the private code part:

https://github.com/owncloud/core/blob/06267fec8fb57ba6921d4c704cb2c05eb1d4c36e/lib/private/defaults.php#L22
https://github.com/owncloud/core/blob/06267fec8fb57ba6921d4c704cb2c05eb1d4c36e/lib/private/defaults.php#L103

cc @michaelstingl @LukasReschke @jancborchardt