Implement modify listener mechanism to GoogleLoginService / make AccountSelector a viewer

This way, multiple viewers (such as `AccountSelector`) of the login state can be synchronized whenever the login state changes.

Note that, however, at the current state, nothing would be broken even if we had multiple `AccountSelector`s some of which are stale; the selector is currently self-sufficient, carrying actual credentials with it. In that sense, the selector is not just a viewer. This issue also suggests to convert the selector to a mere viewer.