[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 checked the iOS behavior. If attributedString is set and a change is font is done, it does affect the attributedString and change it's font size or color.

Android's behavior matches this.

The only thing that is different right now is in iOS, if "attributedString" and "text" is set initially. iOS respects "text". Android respects "attributedString".

I think I'll change this so that Android respects "text" if both are set initially to match iOS parity.