[TIMOB-18062] iOS: AttributedString Parity with Android

Jira: https://jira.appcelerator.org/browse/TIMOB-18062

This PR is a continuation of https://github.com/appcelerator/titanium_mobile/pull/6409/ which came from #6358.

It addresses the comments by @pingwang2011 for the Android portion of the code. The doc is still not completed yet as Titanium.UI.iOS.Attribute and Titanium.UI.iOS.AttributedString yml files are missing the "deprecated".

The propertyAccessors is now used in AttributedStringProxy.java, AttributeProxy.java and LabelProxy.java.
I think this should be the behavior. If a user wants to change the "text" while an "attributeString" is currently being used, the user should create a new "attributeString" with "text" as on of it's value and replace the old attributeString with the new attributeString.

Especially in a case where, for example, where an attributeString is already set with font and backgroundColor in a Label. If a text is set, it the sett font and backgroundColor in attributeString will not be set and used in the Label. This will result in a plain text. If the user wants to change the text and maintain the same font and color, user should replace attributedString and not text. 

I think this should be the behavior.