Contacts gone after rebooting from Safe Mode

[rfc2822]

We should see

• find out what the actual problem is (why does Android remove the contacts),
• if we can do anything against it,
• or at least document it.

Discussed in https://github.com/bitfireAT/davx5-ose/discussions/1038

^{Originally posted by TinfoilSubmarine September 23, 2024} To reproduce:

1. Have address book(s) already synchronized from Nextcloud (30.0.0).
2. Reboot device to safe mode.
3. Reboot again to normal mode.
4. See that address books are not synchronized and must be manually synchronized.