Change PageController#nextPage & #previousPage methods to return Future when the animation completes. (#12299)

* Changed PageController #nextPage & #previousPage to return a Future after completing.

* Removed false statement about returning a Future in PageController#jumpToPage

* Added tests to make sure nextPage & previousPage return Futures.

* Tested that the Futures returned by nextPage & previousPage actually resolve.

* Renaming *pageCalled to *pageCompleted, since that's what it's actually for.