KeyringController does not prevent duplicate accounts properly

[mikesposito]

Current checks for duplicate accounts do not work properly since support for multiple HD keyrings has been added. Moreover, there are several loopholes in the current method in charge of checking duplicates:

• No keyring is checked besides the simple one
• No check is made when adding an account (only when creating the keyring)

The issue can be dangerous because of several selectors that allow picking a keyring based on an account included in it: this can lead to targeting the wrong keyring because of the address belonging to multiple ones (e.g. primary keyring removed instead of the second one)