Add UI to reset active key

[aaroncox]

The snap now uses index 0 to derive the owner key and index 1 (or greater) to derive active key(s).

The owner is never exposed to the user or allowed to sign transactions from external applications.

What this will now do is allow us to create an internal "Reset Active" key function, which is only callable from inside the snap. This hardcoded action when called from the snap will performs the updateauth action, signed using the owner key, to set the active permission to use the default public key from index 1.

This will be useful in some situations, since the user could modify their active permission (using the active key) in external applications, and either add or remove keys control that permission. If the user does this and prevents MetaMask from being able to perform actions with the index 1 key - the user could then call this function to reset their keys to the original state, and regain access to their account.