[TIMOB-18062] iOS: AttributedString Parity with Android

Jira: https://jira.appcelerator.org/browse/TIMOB-18062

iOS deprecation of Titanium.UI.IOS.AttributedString in favor of Titanium.UI.AttributedString. Pull request include Android support for AttributedString from pull request https://github.com/appcelerator/titanium_mobile/pull/6358, which will be closed once this PR is reviewed and merged.

Includes fix for font property in AttributedString, where fontsize was not working.
Add "attributedString" to propertyAccessors.