Show COVID-19 exposure notification statistics

Based on !390 (merged)

Displays how many unique MACs (aka phones) which send exposure notifications have been seen during the last 100 seconds.

It tries to distinguish between Apple and Google devices based on a heuristic mentioned in https://github.com/znuh/microbit-corona-scanner. The user can choose if they want the LEDs and/or the vibration motor to show newly discovered MACs / exposure notifications.

Loosely based on:

• https://github.com/kauzu/CoronaAppDetector
• https://github.com/frankrieger/CoronaAppDetectorM5StickC
• https://github.com/znuh/microbit-corona-scanner

Most of the logic resides in a plain Python script and can be modified by the user as needed.

preload/apps/exnostat/metadata.json

-{"author": "schneider", "name": "Exposure Notification Counter", "description": "Count exposure notifications received via BLE", "category": "Hardware", "revision": 1, "source":"preload"}
+{"author": "schneider", "name": "Exposure Notification Stats", "description": "Count Exposure Notifications received via BLE", "category": "Hardware", "revision": 1, "source":"preload"}