Every call to `keystore::unlock` is followed by `copy_seed()`. The latter
uses a secure chip operation. We can remove that by returning the seed
from unlock and re-using it.

This reduces the number of securechip operations by 1 when:

- when unlocking the device
- showing mnemonic on initialized device
- creating backu on initialized device

This is to reduce the risk of running into Optiga's throttling
security mechanism.