In order to not create an unnecessary `Guest` instance in the
VitalSource container we needed to do the following:

* The `VitalSourceContainerIntegration` is stripped down and converted
  to a class, `VitalSourceInjector`, that does not implement the
  Integration interface but is only responsible for injecting the client
  into content frames.

* The host frame checks the VS frame role and either constructs a Guest
  or sets up the `VitalSourceInjector` as appropriate.

* The `HypothesisInjector`-related logic in `Guest` is extracted out of
  that class and set up in the annotator entry point instead.