Implement AltBeacon protocol

[tthoeye]

Some beacons (those at De Krook) use the AltBeacon protocol. Ideally we adapt the code to be able to detect and use both kind of beacons.

AltBeacon is only available for Android devices.

The Flutter library that is used supports both protocols out of the box, but some adjustments to the code are needed to work with different types of ID, and to provide a fallback for iOS devices.