Fix old key overwrites new key

[ekzyis]

Description

We were always using the old key to decrypt wallets if it was found.

This means that if device sync was enabled before #2169, the existing key could decrypt the migrated wallets.

However, when the passphrase is shown, we encrypt the wallets with a new key. Same is true on passphrase reset.

But on page load, we will still load the old key instead of the new key so the 'unlock wallets' prompt is shown.

This PR fixes this by only loading the old key if a new key was not found.

Checklist

Are your changes backward compatible? Please answer below:

yes

On a scale of 1-10 how well and how have you QA'd this change and any features it might affect? Please answer below:

8. We now always use the new key if it exists. If not but old key exists, we use that. Else we generate a new key.

For frontend changes: Tested on mobile, light and dark mode? Please answer below:

n/a

Did you introduce any new environment variables? If so, call them out explicitly here:

no

[ekzyis]

I'm not sure if this is the bug that siggy47, BlokchainB or OT experienced since they somehow recovered, but afaict, they cannot recover from this bug unless they manually deleted their browser data to wipe the old key and I don't think they did that.