KeyringController: `importAccountWithStrategy` should return imported account address

[mikesposito]

This is part of the effort to consolidate keyring interactions between extension and mobile.
Originally proposed here

Context

On the mobile client, the core KeyringController is patched to return this object:

{
    keyringState: KeyringMemState,
    importedAccountAddress: string
}

Solution

The core KeyringController should adopt the mobile's return object for the importAccountWithStrategy method.