Improve secure storage protection

Current state:

The new (and may also the old plasmo) secure storage protection is based on the user's extensionUnlockPassword. This is a nice idea, but changing the extension unlock password later would be a mess with re-encryption or would require a completely new extension setup.

Recommendation:

Use a secure random key (using secure randomness if possible) as all time encryption key for the secure storage. That key should be encrypted using the extensionUnlockPassword. So when changing the extension unlock password only this single value has to be re-encrypted.