spotify: check cached creds username before use

As proposed at !1183 (comment 1880162)

a nice improvement is if we could check the username for the cached credentials matches any username that was specified. If they don't match then we shouldn't use the cached credentials and instead do a fresh username/password connection.

For example, if you login to account A using usernameA/passwordA and then afterwards login to account B with usernameB/passwordB, it'll automatically reuse the cached creds for account A. You'll have to manually clear out the cached credentials to work around this.