Fix cannot enter SetNodeAddress screen on testnet

This fixes the situation when launching clive in testnet results with crash when entering SetNodeAddress screen because of secret node address not being listed ion backup node addresses.

With this fix its possible to launch testnet and switch between testnet (secret) and mainnet nodes when clive is running.

Now every newly seen address is added to the backup list. But still, the backup list is hardcoded (forced) by us by design and not saved in the profile, so when the address gets changed to other than the one listed in the default backup list, won't be available in the backup list after the clive restart.