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/src/traced/probes/probes_producer.cc b/src/traced/probes/probes_producer.cc
index 5dc47dc..1a3e209 100644
--- a/src/traced/probes/probes_producer.cc
+++ b/src/traced/probes/probes_producer.cc
@@ -249,8 +249,7 @@
   watchdogs_.erase(id);
 }
 
-void ProbesProducer::OnTracingStart() {}
-void ProbesProducer::OnTracingStop() {}
+void ProbesProducer::OnTracingSetup() {}
 
 void ProbesProducer::ConnectWithRetries(const char* socket_name,
                                         base::TaskRunner* task_runner) {