api: allow ShowMnemonic for initialization (allows skipping SD backup)

[benma]

The normal initialization flow is unseeded -> seeded -> initialized:

SetPassword api call to create the seed and set the password -> seeded.

CreateBackup to create the backup onto a microSD card -> initialized.

ShowMnemonic was only allowed in the initalized state, i.e. after a microSD card backup was made.

This commit allows skipping microSD card backups in favor of a mnemonic backup, by allowing the ShowMnemonic API call also in the seeded (not yet initialized) state.

The api call is modified to set the initialized state, and to not ask for the user password when not initialized, which is the same way the CreateBackup call works (which is also allowed after the initialization).

[benma]

Unit tests and a functional test to follow.