Fix problems due to api change between 20 and 21

Hello there,

I am not sure this is the right way to contribute or if you are willing to merge pull request from outsiders. However, while I was experimenting with Transition between Activity, I encountered problems with compiling and running this sample. Indeed, since you upgraded to API 21, it cannot compile neither run.

This is due to changes in API between the L preview and L. This pull request should fix them. At least, it should help other people.

Please, let me know if that is not the right place to do so.