update_engine: Add UMA stats for rollback

Adding the following UMA stats:
- UpdateEngine.Check.TargetVersion: first section of the Chrome OS
  target version set by device policy. Sent during update checks if
  target version policy is set.
- UpdateEngine.Check.RollbackTargetVersion: first section of the
  Chrome OS target version if rollback is also allowed. Sent during
  update checks if both policies are set.
- UpdateEngine.EnterpriseRollback.Success: first section of the
  Chrome OS version to which rollback succeeded. Sent after
  successful installation of the rollback image.
- UpdateEngine.EnterpriseRollback.Failure: first section of the
  Chrome OS version to which rollback failed. Sent after
  installation of the rollback image failed.

Chromium CL of the new histograms: crrev.com/c/1069129

BUG=chromium:843622
TEST='cros_run_unit_tests --board=caroline --packages update_engine'

Change-Id: I0b76fa286498ae0a1830a90034734ed9aa5efd3d
Reviewed-on: https://chromium-review.googlesource.com/1062033
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Marton Hunyady <hunyadym@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
diff --git a/metrics_reporter_android.h b/metrics_reporter_android.h
index 44f770e..06ad8c5 100644
--- a/metrics_reporter_android.h
+++ b/metrics_reporter_android.h
@@ -17,6 +17,8 @@
 #ifndef UPDATE_ENGINE_METRICS_REPORTER_ANDROID_H_
 #define UPDATE_ENGINE_METRICS_REPORTER_ANDROID_H_
 
+#include <string>
+
 #include "update_engine/common/error_code.h"
 #include "update_engine/metrics_constants.h"
 #include "update_engine/metrics_reporter_interface.h"
@@ -33,6 +35,9 @@
 
   void ReportRollbackMetrics(metrics::RollbackResult result) override {}
 
+  void ReportEnterpriseRollbackMetrics(
+      bool success, const std::string& rollback_version) override {}
+
   void ReportDailyMetrics(base::TimeDelta os_age) override {}
 
   void ReportUpdateCheckMetrics(