Migrate to Android Studio

Android Gradle projects use a different directory structure than ADT
Eclipse projects. Here's how the projects were restructured:

* libdanrfid/ => libs/libdanrfid/
* AndroidManifest.xml => app/src/main/AndroidManifest.xml
* assets/ => app/src/main/assets/
* res/ => app/src/main/res/
* src/ => app/src/main/java/