[RKOTLIN-1115] Add support for switching user

[rorbech]

This adds support for switching between logged in users with

public interface App {

    [....]

    /**
     * Switch current user.
     *
     * @param user the user that should be the new current user. The user must be one of the users
     * that are already logged in.
     * @throws IllegalStateException If the [user] is not logged in.
     */
    public fun switchUser(user: User)
}

Closes #1813