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/Android.bp b/Android.bp
index 46ce586..92426f0 100644
--- a/Android.bp
+++ b/Android.bp
@@ -3422,6 +3422,8 @@
     "src/tracing/ipc/posix_shared_memory_unittest.cc",
     "src/tracing/test/aligned_buffer_test.cc",
     "src/tracing/test/fake_packet.cc",
+    "src/tracing/test/mock_consumer.cc",
+    "src/tracing/test/mock_producer.cc",
     "src/tracing/test/test_shared_memory.cc",
     "src/tracing/test/tracing_integration_test.cc",
     "tools/ftrace_proto_gen/ftrace_proto_gen.cc",