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