Keep hidden nav content item hidden after tween completion

[jevakallio]

When using backgroundColor: 'transparent' on Router.headerStyle, after navigating from initial route to a second route, the header content items are displayed one on top of another:

This commit fixes the issue by setting the disappearing nav item's opacity tween end value to 0, instead of 1.

Thanks for all the great work on this component, @t4t5!