Add stubs for UpdateEngine.CleanupSuccessfulUpdate

This API waits for merge to finish after rebooting to
the new Virtual A/B build.

Test: pass
Bug: 138808328
Change-Id: I9422f0ffb1876b71ab45e994fc2d5c76d9ec3a75
diff --git a/update_attempter_android.cc b/update_attempter_android.cc
index 59cdbb8..263498b 100644
--- a/update_attempter_android.cc
+++ b/update_attempter_android.cc
@@ -910,4 +910,11 @@
   return 0;
 }
 
+int32_t UpdateAttempterAndroid::CleanupSuccessfulUpdate(
+    brillo::ErrorPtr* error) {
+  // TODO(elsk): implement b/138808328
+  LogAndSetError(error, FROM_HERE, "Not implemented.");
+  return static_cast<int32_t>(ErrorCode::kError);
+}
+
 }  // namespace chromeos_update_engine