Session verification fails after transfering phones

[stefanceriu]

We're receiving reports that the app stops responding to verification attempts after transfering phones (using apple's own magic transfer flow described here at step 5.

What we believe happens is that:

• the keychain gets transferred over (doesn't in a normal backup as per https://developer.apple.com/forums/thread/93373)
• the app group application support directory (including the crypto db) isn't, possibly because it's in an app group or maybe because we mark it as excludedFromBackup
• the app the uses the token stored in the keychain to restore the old session
• a new crypto db, keys and everything else gets created
• verification stops working as the keys don't match

[pixlwave]

Copying a set of steps from #2169

Steps to reproduce

1. Make an iTunes backup
2. Restore to another device (maybe works with same too, I didn't test)
3. Open Element X
4. Try verifying the new device
5. Verification will always say you cancelled it