Polish code for iOS Task Library Vision

[khanhlvg]

• Avoid using calling the object setters directly. Use property instead. For example, obj.prop = 123
• Change the initialization interface of model output classes to:
  • Have readonly properties. All properties must be set at initialization.
  • For immutable property such as NSString, or object with all immutable objects, it's fine to store the pointer instead of making a copy. This likely applies to most output types of Task Library.
  • Example

[khanhlvg]

@priankakariatyml Could you work on this after finishing the Pod work?

[priankakariat]

I will work on this.