Only permit NFC on Screen1

Only permit NFC to work on Screen1. This is a limitation imposed by
Android, the “singleTask” activity modifier should only be used on a
“Top” activity, which in App Inventor is Screen1.

This change fixes Issue Trac Issue #454, so multiple screens can now
return a value to a previous screen.

Change-Id: I1e07f0ea02721fc4d46e1aa82ab143e6d955efbe