[TIMOB-20501] Android: Simplify defining shared elements for transitions

While I was writing a [blog post](https://github.com/appcelerator-developer-relations/appc-sample-ti520/blob/master/docs/transitions.md) on the new Activity & Shared Element transitions implemented by TIMOB-19374 I was thinking maybe we can make this easier to use. Why does [addSharedElement()](http://docs.appcelerator.com/platform/latest/#!/api/Titanium.UI.Window-method-addSharedElement) require the transitionName as the second argument when it could just read the transitionName property from the view passed as the first argument? Alternatively we could remove the need to set the transitionName on the view in Window A and simply set that property when the view is passed as the first argument of addSharedElement and the name as the second argument. We could support both but that might make it confusion. I'm for removing the second argument.