Update APK generation and follow name conventions

[mariobehling]

Please reduce the number of APKs generated and change the generation of the APK file as follows:

• eventyay-organizer-dev-debug.apk
• eventyay-organizer-dev-debug-fdroid.apk
• eventyay-organizer-master-debug.apk
• eventyay-organizer-master-debug-fdroid.apk
• eventyay-organizer-master-unsigned.apk
• eventyay-organizer-master-unsigned-fdroid.apk