Create a dummy MetricsReporterStub class

This handles the update_engine_sideload target, and thus avoiding
the insertion of "#ifdef _UE_SIDELOAD" everywhere when we report the
metrics for android.

Bug: 30989466
Test: build pass on marlin
Change-Id: Ib00ab8476218df15768bdbae1a3510f8584aa7c7
diff --git a/metrics_reporter_omaha.cc b/metrics_reporter_omaha.cc
index 5f24cbf..0397b83 100644
--- a/metrics_reporter_omaha.cc
+++ b/metrics_reporter_omaha.cc
@@ -112,6 +112,10 @@
     "UpdateEngine.InstallDateProvisioningSource";
 const char kMetricTimeToRebootMinutes[] = "UpdateEngine.TimeToRebootMinutes";
 
+std::unique_ptr<MetricsReporterInterface> CreateMetricsReporter() {
+  return std::make_unique<MetricsReporterOmaha>();
+}
+
 }  // namespace metrics
 
 MetricsReporterOmaha::MetricsReporterOmaha()