Refactor unit-testing of core/service_impl.cc

This just improves unit-testing of ServiceImpl by
introducing MockProducer/ MockConsumer classes.
Also this renames (without adding any new behavior)
the existing methods Producer::OnTracing{Start,StopTracing}
as follows:
- OnTracingStop -> removed as it's currently unsupported
  (see b/77532839)
- OnTracingStart -> renamed to OnTracingSetup, because
  this is what it does. Also this name conflicts with
  Consumer::OnTracingStart, which has a different semantic
  and happens at different times.

Also this CL renames Consumer::OnTracingStop to
OnTracingDisabled to match the {Enable,Disable}Tracing methods.

Change-Id: Ided455d3b37cfefdfbc3eda94e5feccaeeb15a5d
diff --git a/test/end_to_end_shared_memory_fuzzer.cc b/test/end_to_end_shared_memory_fuzzer.cc
index 450e801..a05800d 100644
--- a/test/end_to_end_shared_memory_fuzzer.cc
+++ b/test/end_to_end_shared_memory_fuzzer.cc
@@ -96,8 +96,7 @@
   }
 
   void TearDownDataSourceInstance(DataSourceInstanceID) override {}
-  void OnTracingStart() override {}
-  void OnTracingStop() override {}
+  void OnTracingSetup() override {}
 
  private:
   const std::string name_;