Add scrollToIndex method to ListView

[hshristov]

Both platforms expose methods for scrolling index into view. We should expose such method on the ListView class to allow this functionality.

iOS docs:
https://developer.apple.com/library/prerelease/ios/documentation/UIKit/Reference/UITableView_Class/#//apple_ref/occ/instm/UITableView/scrollToRowAtIndexPath:atScrollPosition:animated:

Android docs (public void smoothScrollToPosition (int position)):
https://developer.android.com/reference/android/widget/ListView.html

[lock]

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