tracing_service_impl_unittest: Mock randomness

Instead of accessing a private member variables.

Change-Id: I2e170b788b5698ac0785b2f0923bb4b4eb7f29fe
diff --git a/Android.bp b/Android.bp
index 8e9dace..65c6cc8 100644
--- a/Android.bp
+++ b/Android.bp
@@ -14898,6 +14898,7 @@
         "src/tracing/service/clock.cc",
         "src/tracing/service/metatrace_writer.cc",
         "src/tracing/service/packet_stream_validator.cc",
+        "src/tracing/service/random.cc",
         "src/tracing/service/trace_buffer.cc",
         "src/tracing/service/tracing_service_impl.cc",
     ],