[TIMOB-13301] iOS support moveNext and movePrevious methods

This adds the moveNext and movePrevious methods to Ti.UI.ScrollableView on iOS

This PR also brings parity to the scrolling behavior of ScrollableView on programmatically changing the current page in iOS and Android. The behavior is as follows

currentPage property - Non Animated Scroll
moveNext,movePrevious,scrollToView methods - Animated Scroll 

Test is attached to [JIRA TICKET](https://jira.appcelerator.org/browse/TIMOB-13301)