[iOS] Fix the notification registration

[NicolasLourenco]

Previously, the method declared on ConnectedDevicesPlatformManager.h was not the same as ConnectedDevicesPlatformManager.m:

• - (void)setNotificationRegistration:(MCDConnectedDevicesNotificationRegistration*)registration; vs - (void)setNotificationRegistration:(NSString*)tokenString

So I've changed to get the device token (string) on .m and create the MCDConnectedDevicesNotificationRegistration inside him.

[msftclas]

All CLA requirements met.