Simplify all stubbing of the REST API in tests by adding a canonical
`stubRestApi` method.
Make sure to never use the <gr-rest-api-interface> element directly
anymore, thus paving the way for converting the element into a service
object and moving into to the services directory.
The reviewer should with looking at all non-test files first.
Change-Id: Ida7c9f0482477a0971fda106f18b8c51e20597c6