Fix: trouble if more than ONE_SHOT showcases are create in the same activity.

The code to chose if show or not the showcase is too early in code. We
factorize it and check later at showing time. This is a common problem
for this library, see https://github.com/Espiandev/ShowcaseView/issues/71.

NOTE: this breaks the ShowcaseViews behaviour, as workaround you have to set explicitly the ``showcaseId``.

This is a dirty workaround for an application I working on, suggestions are welcome.