Overhaul keyring's init method

[mikesposito]

Some methods on keyrings are supposed to be used to initialize them with (or without) certain parameters after construction, like generateRandomMnemonic for HDKeyring or sublitCryptoKey for the QRKeyring.

Some of these can be assimilated to a more generic init function, which is also part of our generic Keyring type which can be implemented by all supported keyrings. We should evaluate if it's possible to remove all these custom initialization methods and have an init method when needed instead.

[desi]

This will likely need to be broken down into multiple issues. @mikesposito