Allow restricting entire classes with @RestrictedApi

[cushon]

Originally requested in #1592

Adding support for restricting API for the entire type. RestrictedApi check currently only supports for annotating methods, with this change, it will be possible to annotate the class and make sure all the methods in that class will be restricted with given annotation configuration.

Before implementing this, we'd need to decide what the semantics of restricting the uses of a type should be. One interpretation is to disallow calls to methods in that type, as if all the methods were annotated with @RestrictedApi. Another interpretation is to try to prevent uses of the type in a more general way, e.g. if it's used as the type of a field, or if it's referenced in another way where the type may not even appear in source (e.g. the return type of foo() in x.foo().bar()).

If you're interested in this feature, it'd be helpful to share detail about use-cases you have in mind, and what behaviour you'd expect from annotating a type with @RestrictedApi.

[yayaa]

Original use case that I had in mind while creating #1592 was that we introduced a new API and it was still under active development so we wanted to restrict the usage of the API to prevent anyone using it without us knowing about it. This was just an additional precaution.

In our case, as the API was simply an interface with a bunch of methods, first interpretation would be already sufficient enough.