Implementation of Apple/Google API

The plan is to switch over the implementation to the Apple/Google Exposure Notification API. This is a meta issue that covers the intent. There are currently some open questions about what is still necessary to get all flows implemented. This issue will be updated accordingly.

Start by reviewing the current implementation and the open questions about the Apple/Google Exposure Notification API. Identify which flows remain necessary and update this meta issue as the implementation plan becomes clear; no specific files or tests are named.

Written by the indexing model from the issue text.